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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
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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.
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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.
Comparative
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.
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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.