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When the systematics of the sipho group of vermetids comes to be fully worked out, the most reliable specific characters will probably be found to be the coloration of the animal and the dentition, as well as the form and sculpture of the nuclear shell. Collections of vermetids should always include if possible colour records of the living animal, as well as preserved material from which radulae and embryos can be obtained. The adult shell sculpture in species of Serpulorbis appears to be in many cases almost identical, and the conchological features show little well-marked difference; the disposition of the whorls is highly irregular according to the shape of the substratum. For the purposes of the present work, which is intended primarily as an account of the structure of the animal, material of Serpulorbis was studied from two Auckland localities, Milford Reef on the eastern shore of Rangitoto Channel, and Otata Island in the Noises Group, five miles north of Rangitoto Island. The Milford material hitherto assigned to sipho yields two clearly defined species, readily distinguishable on the basis of animal coloration and dentition, but apparently without clear-cut conchological differences.
On general grounds it is highly unlikely that the Australian sipho is, properly speaking, represented in New Zealand. Serpulorbis, like other vermetid genera, is a group in which a high degree of speciation is to be expected, both from the sessile habit of the organism and from the mode of reproduction. The free-swimming veliger larval stage is entirely eliminated, and the eggs are retained in thin capsules attached to the inside of the parent shell. There is thus no effective means of transport either of larvae or adults, and the embryo after emergence can wander about for only a relatively short distance before settling in its attached position. The sessile habit renders current fertilization necessary, and eggs are probably fertilized normally by sperm from animals only a few metres distant, a further factor tending to bring about reproductive isolation. It is thus seldom to be expected that single species of vermetids will be represented on both sides of the Tasman. For example, the neozelanic shells ascribed to Lilax nucleogranosum are shown in a forthcoming paper to be separated by valid differences from Verco's South Australian species. Pyxipoma weldii appears to be the only New Zealand exception to this rule, proving to be identical with the Australian and Tasmanian species; this fact may be accounted for by the better facility of distribution in the siliquariids, which are found embedded in buoyant masses of sponge capable of being carried long distances by currents.
In comparing the neozelanic serpulorbids with sipho regard may be had to the existence in the New Zealand “Miocene” of ancestral forms to the present-day species (“Yermicularia” ophioides and “V.” lornensis) [see Finlay, 1927, p. 386] at an era when Australian and New Zealand molluscan relationships were admittedly remote. The sculpture of the adult New Zealand shells is in general close to
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Serpulorbis zelandicus
Fig. 1—Head, foot and pallial cavity of female. The dorsal body wall has been removed
to show the anterior part of the alimentary canal and the pedal mucus gland.
Fig. 2—Diagrammatic view of the head and foot, from above, partly withdrawn into
the shell.
Fig. 3—Diagram of the opening of the pedal mucus duct, pedal tentacles and glandular
tract of the foot. The pedal are shown contracted to approximately half their length.
Fig. 4—Diagrammatic transverse section of pallial cavity and trunk region.
Fig. 5—Transverse section of pedal mucus gland and its duct.
Fig. 6—Transverse section of small portion of mucus gland and two of its smaller ducts.
Fig. 7—Gill filaments in transverse section.
Fig. 8—Terminal region of gill filament, showing arrangement of cilia.
AB.C, abfrontal cilia; A.FT, anterior margin of sole; APC, apical cilia; BL.S,
blood sinus; CT, gill: EXH, exhalant opening of pallial cavity; FR, frontal cilia;
F.T, food tract; G.FT, glandular sole of foot; HY.G, hypobranchial gland;
INH, inhalant opening of pallial cavity; LT.C, lateral cilia; MO, mouth;
MC, mucus cells; MD, median longitudinal duct of mucus gland; MD', Finer ducts
of mucus gland in transverse section; MD”, finer ducts of mucus gland in
vertical section; OE, oesophagus; OS, osphradium; PA, mantle; PD.T, pedal
tentacle; P.G, pedal mucus gland; PG.O, opening of pedal gland duct; PH,
pharynx; R, radula; R.CM, position of tip of radular caecum; R.EP, respiratory
portion of gill; RM, rectum; SK.R, skeletal rod of gill filament; TE, cephalic
tentacle; T.FT, terminal disc of foot.
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Serpulorbis zelandicus
Fig. 9—Stomach and crystalline style caecum, opened from the right side, showing the
course of the ciliary currents.
Fig. 10—Intestine, renal organ and female genital ducts, viewed from the right side.
The renal organ has been opened to show the course of the middle intestine.
Fig. 11—Diagrammatic transverse section of the female genital ducts passing through
the albumen gland and
the receptaculum.
Fig. 12—Diagrammatic transverse section of the capsule gland.
Fig. 13—Portion of terminal lobule of the digestive gland, showing digestive and excretory cells.
Fig. 14—Portion of hypobranchial gland, showing ciliated cells and two types of
glandular cells.
Fig. 15—Portion of epithelium of food tract, showing ciliated and mucus gland cells.
Fig. 16—Portion of epithelium of the capsule gland.
A.CH. anterior chamber of stomach; A.DIV, anterior digestive diverticulum;
CIL.C, ciliated cell; CPS, capsule gland;
CPS', ventral opening of capsule gland; C.ST. crystalline style: DI.C. distal pseudopodial portion of digestive cell;
DI.C, digestive cell with absorbed particulate material; DI.C”, basal portion
of digestive cell, containing greenish spherules before egestion; F, S-shaped fold
of stomach sorting epithelium; EX.C. excretory cell; EX.SPH, excretory spherule;
G.SH, gastric shield; GL.C”, GL.C”, glandular cells of hypobranchial gland;
HY.GL, hypobranchial gland; M.INT, middle intestine; MU.C, mucus gland cell
of food tract; OES, oesophagus; OV.D, ovarian duct; P.CH, posterior chamber
of stomach; P.DIV, posterior digestive diverticulum; P.INT, proximal portion
of the intestine: R.DIG, right (anterior) lobe of digestive gland; REC, receptacu
lumseminis; REN. renal organ; RM, rectum; S.A, ciliary sorting area of stomach;
S,CM, style cnecum; ST, crystalline style: V.TY, ventral typhlosole,
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Fig. 17—Serpulorbis sipho (Lamk.). Mature shell. South Australia.
Fig. 18—Serpulorbis sipho (Lamk.). Juvenile shell after attachment with two spiral
turns. South Australia.
Fig. 19—Serpulorbis aotearoicus n.sp. Egg capsules attached to inner surface of shell
tube of female.
Fig. 20—Serpulorbis aoteoroicus n.sp. Embryo at stage of single-whorled shell. 0.3 mm.
in diameter, removed from capsule.
Fig. 21—Serpulorbis aotearoicus n.sp. Velate embryo at later stage, removed from
capsule, with 1 ½-whorled shell, 0.5 mm. in diameter.
Fig. 22—Serpulorbis aotearoicus n.sp. Embryo at crawling stage, removed from the
mouth of the adult shell tube. The creeping surface of the sole and the concave
operculum are fully developed, and the velum lost.
Fig. 23, 24—Serpulorbis aotearoicus n.sp. The embryo shell at the stage represented in
Fig. 6.
Fig. 25—Serpulorbis sp. Juvenile shell, at similar stage to Fig. 2, Unassigned to species,
Off Oamaru, 50 fathoms, Finlay Collection.
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sipho. As regards the juvenile shell just after attachment, South Australian specimens identified by Mr. B. C. Cotton as belonging to sipho are quite dissimilar in sculpture to young Serpulorbis at the same stage from New Zealand localities. Fig. 25 illustrates a juvenile shell dredged off Oamaru (Finlay Collection). In the adult the sculpture of the young shell is usually obscured by the superposition of the adult coils, and it was not possible to assign the dredged material conclusively to its species. Finally, the coloration of the animal of sipho, illustrated by Quoy and Gaimard (1834) under the name Vermetus arenarius, is again unlike that of either of the New Zealand animals studied. It is therefore proposed to designate the Auckland species of Serpulorbis as follows: