MOLLUSCA 



113 



Family 14. — Olvmdse. 



Genera : OliAia, Brug. ; Ancilla, Lam. ; Harpa, Lam. 

 Family 15. — Volwtidas. 



Genera : Valuta, L. ; Cymbium, Montf. ; Marginella, Lam. ; 

 Volvaria Lam. 



Further Bemarhs on the Reptant Azygobranchia. — Tte 

 very large assemblage of forms coming under this order 

 comprise the most highly developed predaceous sea-snails, 

 numerous vegetarian species, a considerable number of 



Fio. 37.— A. Triton vanegatum, to show the prohoscis or buccal introvert (e) 

 in a state of eversion. a, siphonal notch of the shell occupied hy the siphonal 

 fold of the mantle-skirt (Siphonoohlamyda) ; 6, edge of the mantle-sldrt rest- 

 ing on the shell ; c, cephalic eye ; d, cephalic tentacle ; e, everted buccal 

 introvert (proboscis) ; /, foot ; g, operculum ; A, penis ; i, under surface of 

 the manlile-skirt forming the roof of the sub-pallial chamber. B. Sole of the 

 foot of Fynda tuba, to show a, the pore usually said to be "aquiferous" 

 but probably the orifice of a gland ; &, 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 Lamelhbranchs 

 (fig. 131). 



The surface of the neck is covered by integument forming 

 the floor of the branchial cavity. It has not been cut into. 



Fig. 39. — Animal and shell of PJwrus emt^is. a, snout (not introversible) ; h, 

 cephalic tentacles ; e, 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 and shell of PjiniZoZKDijote. a, siphon ; S, head-tentacles; C, head, the letter placed near the right P^^'Pl® upon exposure tO the at- 

 cye ; tf, the foot, expanded as in crawling ; li, the mantle-slcirt reflected over the sides of the shell. (From Owen, mosphere, and waS USed by the 



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 mc, 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- 

 dium 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 

 the case in all Mol 

 lusca). The heart 

 c lying in the pericardium is seen in close proximity to 



Fro. 40. 



Shell of Calyptrffia, seen from below so as 

 to show the inner whorl &, concealed by the cap- 

 like outer whorl a. 



