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| Fig. 23.—A, Triton variegatum, to show the proboscis or buccalintrovert (e) in a state of eversion. | |
a, Siphonal notch of the shell occupied by the siphonal fold of the mantle-skirt (Siphonochlamyda). b, Edge of the mantle-skirt resting on the shell. c, Cephalic eye. d, Cephalic tentacle. e, Everted buccal introvert (proboscis). | f, Foot. g, Operculum. h, Penis. i, Under surface of the mantle-skirt forming the roof of the sub-pallial chamber. |
| B, Sole of the foot of Pyrula tuba, to show a, the pore usually saidto be “aquiferous” but probably the orifice of a gland; b, medianline of foot. | |
Other glands opening on or near the foot are: (1) The suprapedal gland opening in the middle line between the snout and the anterior border of the foot. It is most commonly found in sessile forms and in terrestrial genera such as Cyclostoma; (2) the anterior pedal gland opening into the anterior groove of the foot, generally present in aquatic species; (3) dorsal posterior mucous glands in certain Cyclostomatidae.
The foot of the Pectinibranchia, unlike the simple muscular disk of the Isopleura and Aspidobranchia, is very often divided into lobes, a fore, middle and hind lobe (pro-, meso- and meta-podium, see figs. 24 and 25). Very usually, but not universally, the metapodium carries an operculum. The division of the foot into lobes is a simple case of that much greater elaboration or breaking up into processes and regions which it undergoes in the class Cephalopoda. Even among some Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot still further carried out by the development of lateral lobes, the parapodia, whilst there are many Pectinibranchia, on the other hand, in which the foot has a simple oblong form without any trace of lobes.
The development of the Pectinibranchia has been followed in several examples, e.g. Paludina, Purpura, Nassa, Vermetus, Neritina. As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes. In fig. 26 the early stages of Paludina vivipara are represented. There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely. A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger. The primitive shell-sac or shell-gland is well marked at this stage, and the pharynx is seen as a new ingrowth (the stomodaeum), about to fuse with and open into the primitively invaginated arch-enteron (fig. 26, F).
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| Fig. 24.—Animal and shell of Phorus exutus. | |
a, Snout (not introversible). b, Cephalic tentacles. c, Right eye. | d, Pro- and meso-podium; to the right of this is seen the metapodiumbearing the sculptured operculum. |
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| Fig. 25.—Animal and shell of Rostellaria rectirostris. (FromOwen.) | |
a, Snout or rostrum. b, Cephalic tentacle. c, Eye. d, Propodium and mesopodium. | e, Metapodium. f, Operculum. h′, Prolonged siphonal notch of the shell occupied by the siphon,or trough-like process of the mantle-skirt. |
In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa. Consequently there is, strictly speaking, no invagination (emboly), but an overgrowth (epiboly) of the smaller cells to enclose the larger. The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally. In such cases the blastopore may entirely close, and both mouth and anus develop as new ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N. Bobretzky, the closed blastopore may coincide in position with the mouth in some instances (Nassa, &c.), instead of with the anus. But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shell-gland, passing through the earlier condition of a trochosphere to that of the veliger. In the veliger stage many Pectinibranchia (Purpura, Nassa, &c.) exhibit, in the dorsal region behind the head, a contractile area of the body-wall. This acts as a larval heart, but ceases to pulsate after a time. Similar rhythmically contractile areas are found on the foot of the embryo Pulmonate Limax and on the yolk-sac (distended foot-surface) of the Cephalopod Loligo. The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore. It is surrounded by a ridge of cells which gradually extends over the visceral sac and secretes the shell. In forms which are naked in the adult state, the shell falls off soon after the reduction of the velum, but in Cenia, Runcina and Vaginula the shell-gland and shell are not developed, and the young animal when hatched has already the naked form of the adult.
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| Fig. 26.—Development of the River-Snail, Paludina vivipara.(After Lankester, 17.) | |
dc, Directive corpuscle (outcast cell). ae, Arch-enteron or cavity lined by the enteric cell-layer or endoderm. bl, Blastopore. vr, Velum or circlet of ciliated cells. dv, Velar area or cephalic dome. sm, Site of the as yet unformed mouth. | f, Foot. mes, Rudiments of the skeleto-trophic tissues. pi, The pedicle of invagination, the future rectum. shgl, The primitive shell-sac or shell-gland. m, Mouth. an, Anus. |
A, Diblastula phase (optical section).