The Struthiolariidae: Reproduction, Life History and Relationships
[Read before the Auckland Institute, May 7, 1949; received by the Editor, September 27, 1949.]
The family Struthiolariidae (Order Mesogastropoda), with two recent neozelanic representatives, is of interest primarily for its importance in New Zealand Tertiary horizons, but also for the feeding habits and mode of life of the living members, as described by the writer (1949). This paper deals with the reproductive system and stages in development of the genera Struthiolaria, and Pelicaria, recognised by Finlay (1927) as radically different but superficially similar stocks which diverged at the beginning of the Miocene. Struthiolaria, with genotype papulosa, is distinguished from Pelicaria, typified by vermis, primarily by the form of the apex, and a knowledge of the ontogeny sheds further light on the nature of the generic separation. A discussion is added on the phylogeny and relationships of the family, based on anatomy and shell features, and especially the examination of preserved material of the primitive living genus Perissodonta, made possible by the kindness of Mr A. W. B. Powell, of the Auckland Museum, to whom the writer is indebted for frequent advice and assistance.
1. The Reproductive Organs
The gonad occupies the same relation in both sexes, forming a massive zone partly investing the digestive gland on the convex side of the visceral spire. The mature testis is brick red in colour, composed of close-set, finely branched spermatogenous tubules, while in the female the ovarian tubules are stouter and shortly lobulate, bright yellow or cream coloured in the adult. The immature ovary—in specimens in which the shell lip is not yet callused—forms a translucent jelly-like layer in which the maturing ova are scattered as tiny opaque specks. The genital ducts in both sexes show a number of primitive characters, the ciliated seminal groove in the male remaining unclosed as far as the tip of the penis, and the prostate region showing the simplest structure. In the female a ciliated genital furrow is retained corresponding exactly with the seminal groove in the male, while the capsule gland or distal portion of the glandular oviduct remains unclosed along its whole length. In the following account, reference is made primarily to Struthiolaria papulosa, but applies equally well to Pelicaria vermis except where the latter is separately mentioned.
i. The Male Genital Ducts
The male duct is divided into a closed posterior region, the gonadial duct, which receives tributary ductules from the testis, and a long seminal groove running forward along the right side of the trunk, and continuing to the tip of the cephalic penis. The gonadial duct is
wide and thin-walled, running along the concave surface of the visceral mass, immediately beneath the renal organ, and distended throughout the year in the adult with seminal fluid, to form a close mass of opaque white convolutions. It functions as a vesicle for the storage of sperm, and is lined with short-celled cubical epithelium, with an underlying white fibrous coat. Eupyrenic sperm heads become periodically attached in a continuous row to the epithelium, and various stages of sperm disintegration are found within the cytoplasm.
The gonadial duct opens into the seminal groove at the posterior end of the mantle cavity. The first portion of the groove, about 1 cm. in length, represents a “prostate” of very elementary character, of the type described in primitive mesogastropods by Fretter (1946). It is bounded by tall folds of the pallial wall and the floor is thrown into several smaller epithelial ridges. The lining cells are columnar (20μ) with a dense coat of fine cilia reaching 12μ in height. There are frequent gland cells with darker staining basal nuclei and clear, non-staining secretion contents. Subepithelial glandular lobes as found in the prostate of more advanced mesogastropods are not developed. The seminal groove anterior to the prostate is enclosed beneath the right margin of the integumentary food groove. Gland cells are here less frequent, and the columnar epithelium shorter, though the cilia remain equally long. The penis is a long, recurved column, rather flattened, which in the adult may reach a length of 6 cm. In the immature animal it is a small pointed papilla.
An interesting feature in the Struthiolariidae is the extreme development of sperm dimorphism. The eupyrenic sperms, which are much the more numerous, are typical in appearance, with deeply staining rod-like heads (8μ) and long flagella (40–50μ). The oligopyrenic or vermiform sperms are giant cigar-shaped structures in Struthiolaria, 95μ–100μ in length. In Pelicaria they are shorter (50μ) and of curved, falciform shape. They progress by regular undulating waves of the flattened-marginal membrane, and appear to be formed by the elongation of spermatocytes that have lost their chromatin by unclear disintegration.
ii. The Female Genital Ducts
The female genital tract commences posteriorly with a narrow renal oviduct, receiving branched ovarian ducts and opening forward into a glandular region forming the “pallial oviduct” of Fretter, consisting of an albumen gland and a capsule gland, together with a large saccular receptaculum seminis. The ciliated oviducal groove—representing the seminal groove of the male—carries the ova forward into an incubatory pouch, situated anteriorly upon the right side of the mantle.
The renal oviduct (Text Fig. 1 OD.) is a thin-walled tube visible superficially on the right mantle wall and recurving to open back into the posterior end of the albumen gland. It is non-glandular, lined with ciliated columnar cells, with an underlying investment of connective tissue without muscle fibres. The epithelium is thrown into several longitudinal folds which increase in height anteriorly so as to form a valve closing the albumen gland opening. Further back the cytoplasm is densely granular and absorbs particles of yolk.
Neither in the male nor the female Struthiolaria does a genitopericardial duct exist. The albumen gland, capsule gland and the receptaculum seminis lie on the right posterior mantle wall, just below the rectum and are conspicuous by their yellow colour. The albumen gland (alb.) is a short, curved tube, about. 0·7 cm. in length, with the lateral walls thickened and glandular, enclosing a slit-shaped dorsoventral lumen, bounded above and below by narrow epithelial strips suturing the gland lobes. The lateral walls are divided into about 20 separate lobules, giving the tube a regularly annulated appearance. The histology of the glandular lobes in both the albumen and the capsule glands agrees closely with the description by Fretter (1942) for Stenoglossa: numerous spherical gland cells are packed together, with long tails opening as cell ducts through the ciliated epithelium bordering the lumen.
Fig. 1–Struthiolaria papulosa. Female genital ducts viewed from the right side, with portion of string of egg capsules. X 6
Fig. 2–Struthiolaria papulosa. Diagram of transverse section through the female ducts at the level of the receptaculum duct.
Fig. 3–Pelicaria vermis. Female genital ducts viewed from the right side, with egg capsule. X 6.
Fig. 4–Aporrhais pes-pclicni. Similar view of female genital ducts.
The capsule gland (cps.) in Struthiolaria is a narrow, almost straight tube, about twice the length of the albumen gland, which it resembles in general structure. The lumen is primitively open along the ventral side, and is confluent with the proximal part of the ciliated oviducal groove. The right wall is attached to the mantle, while the left hangs freely, carrying along its margin a narrow flange of integument, which may be temporarily approximated to the right side to close the lumen. Through the tube so formed the penis is introduced as far as the opening of the receptaculum duct, which forms a small slit in the ventral wall of the albumen gland, just behind its junction with the capsule gland. The receptaculum (rec.) is closely applied to the inner aspect of the albumen gland, forming a large reniform sac, bulging deeply against the renal organ. Its convex dorsal edge is crested with small sacculations, while from the straight ventral margin arises anteriorly the short wide receptaculum duct. The receptaculum has a thick, circular muscle coat (60μ–70μ), and is lined with tall, clear-staining columnar epithelium, 70μ in height, with a short ciliary coat (5μ) usually obscured by the attachment of a row of eupryenic sperm heads. During the disintegration of surplus sperm, the epithelium becomes markedly glandular, with large, clear-staining goblet cells, whose secretion has apparently a digestive action on the sperms. As distinct from the Stenoglossa (Fretter, 1942), sperm ingestion does not occur.
A large amount of sperm is stored in the receptaculum at one copulatory act, and the receptaculum duct which is strongly ciliated, with well-developed muscular walls, acts as an effective sphincter, releasing backwards small amounts of sperm along the ventral groove of the albumen gland. Fertilisation doubtless occurs at the proximal end of this gland, before the egg receives its secretion layers. The eggs are rotated by the ciliated epithelium of the albumen gland, where two types of substance are secreted—a coating of albumen from the cells of the right lobe, which are finely granular, staining darkly with van Giesen's, and an admixture of mucus from the more coarsely granular cells of the left lobe, which show a strong affinity for haematoxylin. The capsule gland cells are all of the same type, staining deeply with haematoxylin, and secreting a thin, tough, mucoid capsule round each cluster of 18–24 eggs (Text Fig. 1). Each egg is at this stage 0·3 mm. in diameter, and the capsules are spherical, 2 mm. long. The capsule gland is of sufficient length to contain six or seven capsules end to end at one time, and the contents pass forward along the ciliated oviducal groove to enter the incubatory pouch in a continuous string.
The incubatory pouch is a spacious oval pocket, about 3 cm. in length, bulging deeply into the pallial cavity, and yellow in colour from the contained eggs. The aperture is an oval slit at the posterior end, through which the egg strings are transferred, by the retraction of the head and foot into the mantle cavity, so as to approximate the termination of the oviducal groove to the mantle wall. At the extrusion of the veligers the mantle margin is extended over the rim of the shell, so as to bring the pouch aperture to an external position, when the contents are expelled by wave-like muscular contractions of the pouch wall.
II. Life History i. Struthiolaria papulosa
As in most incubating molluses, reproduction probably takes place all the year round, ovigerous females being collected at Auckland during November, December, April, June, August, and September. As many as 1,000 to 1,500 capsules are contained in the pouch, and a total of 20,000–30,000 veligers are thus liberated from a single brood. An early stage in development removed from the pouch (Pl. 55, Fig. 2) represents an abbreviated two-lobed veliger, approximately 0·5 mm. in length, with a pair of short, undivided velar rudiments projecting laterally. The visceral mass is opaque and yolky, covered by a shell rudiment, in the form of a delicate chitinous cap, while the head region forms a large spherical protuberance with neither eyes nor otocysts visible. The foot is present as a short, tongue-shaped lobe, behind the head, and the embryos move about freely within the capsule, mainly by muscular movements of the velar lobes.
At the final stage in incubation, when the capsules have attained approximately twice their original size, and the large four-lobed veligers can be clearly identified within, the pallial margin is widely extended over the lip of the shell, bringing the pouch aperture to an external position, and a string of capsules is extruded, the veligers at once breaking through the capsule membrane and commencing to swim freely. The expanded velum (Pl. 55. Fig. 1 VL.) now measures 1 ½ 2 mm. across, deeply subdivided to form a pair of large swimming lobes at either side. Near the tip of each lobe is a very conspicuous yellowish-brown spot, from which a yellow band extends around the velum just inside the margin. The thickened rim of the velum is beset, with stiff, robust, cilia, approximately 15μ in length, which serve, together with movements of the velar lobes themselves, for the rapid propulsion of the larva. The larval shell (sh) is at this stage 0·3 mm. in height and a little less in width, chitinous, perfectly transparent, without sculpture and forming a tiny wide-mouthed cup. with a short, single-whorled planorboid coil. The foot (ft) lying behind the velum is narrow and squarish along the anterior edge, becoming wider and oval behind, the whole surface glandular and finely ciliated. No opercular rudiment is yet present. The cephalic tentacles are short and clubshaped, tipped with several longish sensory cilia. The eyes are prominent at the tentacle bases, and the otocysts distinctly seen within the head cavity. The etnidial filaments are not yet elongated as in the adult, and ciliary feeding apparently does not occur at this stage.
The alimentary canal (Pl. 55, Fig. 3) shows essentially its adult form. The buccal bulb with its minute radular rudiment leads into the narrow, strongly ciliated oesophagus, passing back to the stomach—a transparent globular sac, provided with a short caecum from which projects a perfectly developed crystalline style, rotating clockwise 40 times a minute, with a mucus-bound mass of food material attached to the gastric end. The two digestive gland lobes are filled with brownish yolky contents, and there is a two-way pasage of particles along the diverticula leading out of the stomach. The intestine is a narrow ciliated tube encircling the renal organ, and continuing to
the mantle edge as a short, wide rectum. The kidney (Pl. 55, Fig. 3 kd), which as in all marine prosobranchiates gives rise directly to the adult renal organ, is a large flattened or spherical sac on the right pallial wall, extending forward almost to the mantle margin. Its large hexagonal lining cells each contain a dark granular spherule somewhat resembling a nucleus, and the sac undergoes regular pulsatile movements. The two-chambered definitive heart (ht) lies in the pericardium and there is as well a pulsatile larval heart or thoracic vesicle (lh) formed by a dilation of the anterior aorta beneath the pallial cavity floor, which projects in diastole from the aperture of the pallial cavity.
The later development of Struthiolaria papulosa has not been worked out. From the evidence of the 2 ½-whorled protoconch, and the extremely well-developed swimming organs, it seems clear that there is a fairly long stage of planktonic existence. Tow net samples taken at Cheltenham (2.49) yielded one veliger that from size, shell sculpture, and general appearance could be tentatively recognised as a Strutiolaria at the 2-whorled stage, towards the end of larval life. The velum was damaged and withdrawn, although, from the striking resemblance of the four-lobed larva to that of the related Aporrhais, it may well be that, as in the latter genus, the final stage larva of Struthiolaria possesses a six-lobed velum.
ii. Pelicaria vermis
In the genus Pelicaria the embryos are completely incubated in the pouch, and are liberated as tiny replicas of the adult which at once take on a benthic existence. The female reproductive apparatus is in general closely similar to that of Struthiolaria. The albumen gland is, however, shorter, and relatively stouter, the receptaculum forms a smaller bulbous sac, while the capsule gland is relatively more spacious, a single elongate egg capsule occupying its whole length. Only seven or eight capsules are contained in the pouch at one time, each 7 mm. long, elongate-ovoid, not joined in a string, and each containing five eggs, approximately 1·0 mm. across and white in colour (Text Fig. 3). Liberation of the embryos normally takes place when the shell is 4 ½–5 mm. in length, with two rounded whorls and the typical apex of this genus. A thin-walled, elongate, capuliform bulb (Pl. 55, Fig. 4) is placed transversely to the long axis of the shell, being at first transparent and entirely chitinous. It is followed by a calcified spirally coiled region sculptured with fine spiral cinguli from the beginning of the second whorl. As may be seen, this apex differs markedly from the small planorboid tip in Struthiolaria. On liberation, the embryo (Fig. 6), which is translucent milk-white in colour, at once begins in crawl about and to execute the characteristic movements of the adult. The sole is long and narrow, with an opercular spine present by which the typical “righting” movement of the adult can already be performed.
Plate 55, Fig. 5, represents an embryo removed from the pouch at an earlier stage of development. The shell measures 1 ½–2 mm. in length, consisting of a capuliform apex and a single finely spirally sculptured whorl. These embryos move about actively within the capsule, by the action of the foot, and finally break through the enclosing membrane by the use of the opercular spine. A large vestige
Fig. 1—Struthiolaria papulosa. Four-lobed veliger larva after liberation from the incubatory pouch, X 30.
Fig. 2—Struthiolaria papulosa. Earlier (two-lobed) veliger removed from the incubatory pouch, X 35.
Fig. 3—Struthiolaria papulosa. Enlarged sketch of veliger at stage shown in Fig. 1, illustrating structural details.
Fig. 4—Pelicaria vermis. Apex of adult shell, intact (above) and broken, with calcareous septum developed (below).
Fig. 5—Pelicaria vermis. Embryo with rudimentary velum, during incubation in the pouch, X 30.
Fig. 6—Pelicaria vermis. Embryo after liberation from the pouch, at the beginning of the benthic stage, X 8.
AP. apex; CE, cerebral ganglion; CT, ctenidium; DIG, digestive gland; F, foot; HT, definitive heart; KD, kidney; LH, larval heart or “thoracic vesicle”; M, mouth; NR. nerve ring; OP, operculum; OT. otocyst; PA. pallial margin; PR, proboscis; PT, pallial tentacles; SH, shell; SM. stomach; ST, style caecum; V, velum; VL, velar lobe.
of the velum is retained in the form of a transverse rectangular flap (vf) surrounding the head region in front of the foot, with a rim of marginal cilia keeping up an active beat. There is no coloration as on the velar lobes of Struthiolaria, and the organ is lost completely before the embryo normally emerges. The gill has developed the same proportions as in the ciliary feeding adult, and a long fringe of free, rod-like filaments generally projects obliquely forward over the margin of the mantle. The renal organ resembles that of the adult, but the larval heart is still prominent. The foot is oval behind, with an opercular spine developed and two broad tracts of gland cells along the sole. Its anterior end is squarish and strongly ciliated, incised along the edge by a deep transverse groove.
In laboratory tanks are embryos frequently liberated at this early stage of development, enclosed in the capsules, which fact may indicate a fairly recent acquisition of the incubating habit. It was not discovered how well equipped these velate embryos may be for free existence, though they begin to feed as soon as they emerge from the capsule. Certain ciliary currents appear to be adapted for food collecting. There is a strong rotatory current along the food groove, though detrital contents were not seen. Immediately on liberation, however, a strong ciliary current was observed to carry a string of fine plant debris along the groove of the anterior edge of the foot, which was always held turned upwards close to the mouth. Periodically the string was ingested, and it seems that at the earliest free-living stage the ciliated groove of the foot may replace the food groove in collecting nutriment.
The female genital ducts of the related Aporrhais pes-pelicani were dissected for comparison with Struthiolaria, and were found to agree closely in general plan, though there are a number of differences in detail. The albumen gland (Text Fig. 4 alb) is a smooth-walled, curved tube receiving anteriorly the duct of a spacious cylindrical receptaculum (rec) lying for the most part below the albumen gland. The capsule gland (cps) is very short, remaining ventrally unclosed, and leading into a ciliated genital groove, which runs forward as in the strombids as far as the anterior edge of the foot. There is no incubatory pouch, and Lebour (1933) has recorded the deposition of eggs 0·24 mm. across, laid singly attached to sand grains or debris. The first free-swimming stage is a two-lobed veliger, metamorphosing to a four-lobed larva closely resembling in detail the corresponding stage in Struthiolaria, even to the brown spot at the end of each velar lobe.
Aporrhais, Struthiolaria, and Pelicaria thus show an evolutionary series in reproductive habits: in Aporrhais single eggs are deposited separately and the larval history is unabbreviated; in Struthiolaria a large number of fairly small eggs is enclosed in a common capsule, and partial incubation takes place, with the retention of the young to the four-lobed veliger stage; in Pelicaria the eggs are reduced in number and large and yolky, the free-swimming stage being wholly eliminated and the embryos liberated at the benthic stage.
The Struthiolariid Apex
The significance of the apex difference in Struthiolaria and Pelicaria, discussed by Finlay (1931), becomes fairly clear with the elucidation of the life history. Struthiolaria possesses a small, close-coiled, multispiral, calcareous protoconch as typified by S. papulosa. In Pelicaria the tip of the shell forms originally a horny, capuliform bulb, as seen in the fragile apex of the pouch embryo. In the course of development, this chitinous coating is gradually filled up with a structureless calcareous deposit as the animal recedes from the extreme tip, until in the adult a solid blob is formed, sometimes narrowly perforated. The horny coating is soon rubbed off during development, while in a large number of cases the fragile bulb may be detached or damaged at an early stage, and the tip of the shell is then plugged by a similar blob of shelly material deposited within the broken edge, a short distance down from the original apex. As Finlay has surmised, the Pelicaria apex of the scaphelloid type is clearly derived from the multispiral, calcareous type of Struthiolaria. The difference is probably due simply to the increase in the size of the egg in species in which the embryo is incubated. In those forms where there is a long free-swimming stage, the egg is relatively small, and the shell apex is first formed as a tiny cap covering the visceral mass of the embryo formed from the yolky hemisphere of the egg. Where incubation occurs the embryo must live for some time on its yolk reserves, and the egg is much more bulky, with the visceral cap correspondingly large, and of capuliform shape. Though, as Finlay lays down, the multispiral and paucispiral forms will generally be found to be entitled to separate generic status, the distinction may not always be of fundamental importance as a taxonomic character. Thus, for example, in the case of Littorina saxatilis. Seshappa (1947) has drawn attention to the existence of oviparous and ovoviviparous races, apparently characterised by apex differences, within a group classically regarded as forming a single species.
It was suggested by the present writer (1949)—after a comparison of the biology, mode of feeding, and digestive systems—that the Struthiolariidae are derived from an aporrhaid stock. The Aporrhaidae are geologically the oldest group of the Stirps Strombacea, as recognised by Thiele (1931). They are structurally more primitive than either the Struthiolariidae or the Strombidae, and would appear to have given risen separately to both of these families. Marwick and Finlay (1935) had previously, from a study of shell characters, postulated an aporrhaid origin for the Struthiolariidae, citing the New Zealand genus Struthioptera as the probable derivation point. A knowledge of reproduction and development now lends further support to this view.
The structural relationships of the three families—Aporrhaidae, Strombidae, and Struthiolariidae—were briefly discussed by the writer (1947) and it was concluded that the common ancestral form must have fairly closely resembled a modern aporrhaid, save that it no doubt crawled upon the surface of soft sand or mud and was not specially adapted for burrowing. The modern Strombidae are the
farthest removed group: while still closely resembling the Aporrhaidae in the nervous system, they have become highly specialised in the digestive system (Yonge, 1932) and in the female reproductive organs and often grotesquely modified in external features. The aporrhaid-struthiolariid line diverged from the basal stock by the adoption of a burrowing habit, with the formation of siphonal tubes by the elongated proboscis; the pharynx and radula are reduced owing to the mode of feeding on finely divided plant detritus. The Struthio-lariidae have proceeded a good deal further in their evolution than the aporrhaids, most notably in the following features:
Development of ciliary feeding, with associated adaptations of the pallial organs.
Reduction of the buccal and oesophageal portions of the gut, with further diminution of the radula, vestigiality of the salivary glands, and loss of the mid-oesophageal crop.
Specialisation of the nervous system, with incorporation of the subintestinal ganglion in the nerve ring.
Development of the incubatory habit, with the formation of a pallial brood pouch.
Marwick and Finlay (supr. cit.) suggest that Struthioptera gave rise to Conchothyra, which includes the earliest shells assigned to the Struthiolariidae. The next step is represented by Struthiolarella, which would then “appear as an ancient basic member of the Struthiolariidae.” Radular and opercular material first becomes available for the family' with the two species of Perissodonta, the most primitive living members of the group, which show a widely discontinuous geographical distribution, P. mirabilis occurring at Kerguelen Land and P. georgiana at South Georgia. They probably, however, represent true relict forms of an ancient stock, since both agree in the peculiar multiplication of the marginal teeth of the radula. The shell agrees most closely with Struthiolarella, with which genus Steinmann and Wilckens (1908) proposed to incorporate Perissodonta. Marked aporrhaid features are retained, such as the strongly rounded body whorl; the strong, sigmoidally curved axial ribs, especially well developed on the older whorls; the unisinuate outer lip receding at the suture to form a wide posterior sinus; and the straight columella. The radula (Text Fig. 5) is also reminiscent of Aporrhais: the central tooth shows the transition from a row of sharp denticulations to the large, multi-serrate cusp of the modern struthiolariids, while the laterals have the triangular aporrhaid shape as distinct from the rectangular form in Struthiolaria and Pelicaria. However, the increase of the marginals to five pairs is an anomaly which indicates that Perissodonta has diverged some distance from the main stem of the family.
The operculum of Aporrhais pes-pelicani (Text Fig. 5) forms an oval dise, with a callused rim surrounding the muscle scar, and the distal end produced into a wide, squarish plate. The struthiolariid operculum (Text Figs. 6 and 7) is derived by the production of the callus rim to form a sharp distal spine—an adaptation for the use of the operculum in assisting locomotion by thrusting into the substratum, In Perissodonta (Text Fig. 6) the operculum is of the clawed
struthiolariid type, but like the radula diverges from the main evolutionary line, the disc being strongly angled along the axis of the spine.
The animal of Perissodonta is a typical struthiolariid: the gill filaments are elongate and linear, and there is a food groove, suggesting that ciliary feeding takes place. Unfortunately, the four specimens examined were all males, and it is not yet certain whether or not a brood pouch is present. The shell apex is small and planorboid, as in Struthiolaria, suggesting that there is a free-swimming stage of some duration.
Among the modern struthiolariids, Pelicaria is clearly an advanced genus which, according to Marwick (1923), diverged from Struthiolaria in the late Oligocene or early Miocene, is scantily represented in the Miocene, and exhibits a marked speciation in the Pliocene. The salient diagnostic feature is the capuliform apex, and in addition Finlay (1927) has pointed to the well-marked shell characters distinguishing Pelicaria from Struthiolaria: the presence of strong, spiral cinguli giving a bicarinate or tricarinate appearance to the whorls: the frequently rounded body whorl: the canaliculate suture line: the lesser development of the inner lip callus and the almost straight outer lip. The radula (Text Fig. 8) is very close to that of Struthiolaria (Text Fig. 7), but has a highly distinctive feature in the exaggeration of the cusps of the central and lateral teeth. The central cusp is very long and well overlaps the succeeding tooth.
Restriction of range in Pelicaria at an early stage, consequent upon the retention of the veliger, may well have been a contributing factor in the more complete separation of the genus. Ecological isolation possibly played a part in the original divergence of Struthiolaria and Pelicaria, the latter showing at the present day a greater tolerance of muddy conditions, although there is no apparent adaptive difference and the two forms very generally overlap. The effect of delay in liberation of the larvae is clearly seen in the restricted geographical range of the existing struthiolariid genera, as compared with the wide range of Aporrhais in the northern hemisphere. It is significant that Pelicaria vermis is confined to the North Island of New Zealand, while Struthiolaria papulosa extends throughout New Zealand and reaches the Kermadecs. The pronounced Pliocene radiation of species of Pelicaria may similarly be ascribed to geographical isolation following the elimination of the larval stage, while the tendency of the recent Struthiolaria papulosa to break up into local races may be also due in part to the shortening of the free-swimming phase. It may be remarked, however, that the so-called geographical “races” of S. papulosa do not appear to be always clear-cut: for example, by no means all of the Foveaux Strait specimens are of the smooth shouldered “gigas” type, while numerous examples of the short, moderately nodulose shell, of the topotypic Cook Strait form have been collected from Cheltenham, Auckland.
It is interesting to note that Tylospira scutulata, the single living Australian representative of the Struthiolariidae, has a bulbous apex similar to—but rather smaller than—that of Pelicaria. The animal
and life history of Tylospira are unknown, but from consideration of sculpture characters Finlay (1931) places the genus fairly close to Pelicaria. It is certainly a highly advanced, gerontic form, and it would be desirable to examine the radula and operculum—which the writer has not so far been able to procure—in order to determine the exact relation of the genus to the main stem of the family. It is at least a possibility that the scaphelloid apex may be a parallel development in Pelicaria and Tylospira, by the independent loss of the free swimming stages.
The following phylogenetic diagram expresses the relationships of the Struthiolariidae, as suggested from the geological evidence, and the anatomical results presented above.
References To Literature
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5.— 1940. The Genital Ducts of Theodoxus, Lamellaria and Trivia, and a Discussion on their Evolution in the Prosobranchs. J. Mar. Biol. Assoc. U.K., 26, 312–349.
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13. Yonge, C. M., 1932. Notes on Feeding and Digestion in Pterocera and Vermetus, with a Discussion on the Occurrence of the Crystalline Style in the Gastropoda. Sci. Repts. Gt. Barrier Reef Exped., 1928–29, Brit. Mus. (Nat. Hist.), I, 259–281.