Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/678

Rh 650 MOLLUSC A [AZYGOBRANCHIA. Family 14. Olividse. Genera: Oliva, Brug. ; Ancilla, Lam.; Harpa, Lam. Family 15. Volutidie. Genera : Voluta, L. ; Cymbium, Montf. ; Margindla, Lam. ; Volvaria Lam. Further Remarks on the Replant Azygobranchia.Thz very large assemblage of forms coming under this order comprise the most highly developed predaceous sea-snails, numerous vegetarian species, a considerable number of f Fio. 37. A. Triton variegatum, to show the proboscis or buccal introvert (e) in a state of eversion. , siphonal notch of the shell occupied by the siphonal fold of the mantle-skirt (Si phonochlamyda) ; b, edge of the mantle-skirt rest ing on the shell ; c, cephalic eye ; d, cephalic tentacle ; e, everted buccal introvert (proboscis) ; /, 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 o, the pore usually said to be &quot;aquiferous&quot; but probably the orilice of a gland ; b, median line of foot. fresh-water, and some terrestrial forms. The partial dis section of a male specimen of the Common Periwinkle, Littorina littoralis, drawn in fig. 46, will serve to exhibit the disposition of viscera which prevails in the group. retractor muscle of the foot, which clings to the spiral column or columella of the shell (see fig. 42). This col- umella muscle is the same thing as the muscular surface marked c in the figures of Patella, marked k in fig. 91 of Nautilus, and the posterior adductor of Lamellibranch.s (fig. 131). The surface of the neck is covered by integument forming the floor of the branchial cavity. It has not been cut into. Fro. 39. Animal and shell of Phorus exutus. a, snout (not introversible) ; &amp;gt;, cephalic tentacles ; c, right eye ; d, pro- and meso-podium, to the right of this is seen the metapodium bearing the sculptured operculum. Of the organs lying on the reflected mantle-skirt, that which in the natural state lay nearest to the vas deferens on the right side of the median line of the roof of the branchial chamber is the rectum i, ending in the anus a. It can be traced back to the intestine i near the surface of the visceral hump, and it is found that the apex of the coil formed by the hump is occupied by the liver h and the stomach v. Pha rynx and oesophagus are con cealed in the head. The enlarged glandular structure of the walls of the rectum is frequent in the Azygobranchia, as is also though not universally the gland marked y, next to the rectum. It is the adrectal gland, and in the genera Murex and Purpura secretes a colourless liquid which turns FIG. 38. Animal ami shell of Pyntlalxvigata. a, siphon ; b, head-tentacles ; C, head, the letter placed near the right P ur pl e upon exposure to the at- eye ; d, the foot, expanded as in crawling ; h, the mantle-skirt reflected over the sides of the shell. (From Owen, mosphere and was USed by the L _ 1 1 1 1 i r 11,1 * &quot; The branchial chamber formed by the mantle-skirt over hanging the head has been exposed by cutting along a line extending backward from the letters vd to the base of the columella muscle me, and the whole roof of the chamber thus detached from the right side of the animal s neck has been thrown over to the left, showing the organs which lie upon the roof. No opening into the body-cavity has been made; the organs which lie in the coiled visceral hump show through its transparent walls. The head is seen in front resting on the foot and carrying a median non-retractile snout or rostrum, and a pair of cephalic tentacles at the base of each of which is an eye. In many Gastropoda the eyes are not thus sessile but raised upon special eye-tentacles (figs. 43, 69). To the right of the head is seen the muscular penis p close to the termination of the vas deferens (sper matic duct) vd. The testis t occupies a median position in the coiled visceral mass. Behind the penis on the same side is the hooklike columella muscle, a development of the ancients as a dye. Near this, and less advanced into the branchial chamber, is the single renal organ or nephri- clium r with its opening to the ex terior r . Internally this glandular sac presents a second slit or aperture which leads into the pericardium (as is now found to be flip r&amp;gt;a&amp;lt;w in oil &quot;Ur.1 Fl - 4 ~ Sllp11 of Cal yP tra&amp;gt;a. seen from below so as to show the inner whorl b, concealed by the cap- lusca). The heart like outer whorl c lying in the pericardium is seen in close proximity to