104 



COMPARATIVE ANATOMY 



CHAP. 



of the evaginated proboscis three pairs of conical processes (cephalic cones), with 

 special nerve endings and glands whose sticky secretion helps in the capture of prey 

 (Clionidce). 



4. The proboscis may be wanting. There is then on each side of the mouth a 

 long extensible buccal appendage carrying at its base the labial tentacle. 



Pteropoda thecosomata. The head is, as a rule, not sharply separated from the 

 body, and has no invaginable snout, but one pair of tentacles which answer to 

 rhinophores, and sometimes lie in sheaths at their bases. The left tentacle may 

 become rudimentary. In the Thecosomata the male copulatory organ lies on the 

 upper side of the head, near the tentacle. 



3. Pulmonata. 



The head is here distinct from the foot ventrally, but passes dorsally into the 



neck. It carries two or four tentacles. The Stylom- 

 liialophom, which are terrestrial, have four tentacles 

 (Fig. 97), an anterior and a posterior pair. The 

 posterior, which are usually the longer, carry the eyes 

 on their tips. The tentacles are hollow tubes filled 

 with blood and connected with the blood spaces of 

 the head. They can be invaginated from the very 

 tip into the head, special muscles acting as retractors 

 which, when the tentacle is evaginated, run from the 

 head to the tip of the tentacular cavity. 



The Basommatophora, which are aquatic, have 

 only one pair of tentacles which are usually triangular 

 FIG. or. Helix, front view, creep- and flat. They are solid, and not invaginable, but 



in- with extended tentacles (after merely contractile. The eyes lie on the inner side of 



Howes), s, Shell ; t lt optic tentacle ; t ] le j r l jases> 



t, anterior tentacle; m, mouth ; ?i, , . 7 ,~, ,. ., 



^ j ips In certain Pulinonata (G-landinu, Zonites, Onei- 



diiiin) the upper lip may be drawn out into a lobe or 



labial palp on each side. ThisUabial palp in Glundina can move very freely, and is 

 the seat of a fine sense of touch. 



On the right, behind the right tentacle, lies the common genital aperture, or, in 

 cases where the male and female apertures are distinct, the male aperture. 



B. Seaphopoda (Fig. 101, p. 113). 



In this order the non-invaginable snout is ovoid or barrel-shaped, 

 and projects from the body, over and in front of the foot, downwards 

 into the mantle cavity. At its extremity lies the mouth, surrounded by 

 a circle of dentate oral lobes shaped like oak-leaves, four on each side. 



At the boundary between the bases of the foot and of the snout, 

 to the right and left of the cerebral ganglion, a shield-shaped lobe 

 rises from the body on each side ; this is attached, at the centre of its 

 inner side, by a short slender stalk to the body wall, concrescence also 

 taking place at its lower edge. This shield carries numerous filamentous 

 or vermiform glandular tentacles, which move very freely and can be 

 protruded far beyond the mantle aperture. 



The ends of the tentacles are swollen into the shape of a spoon, and can become 

 attached to foreign objects like suckers. Each swelling has long ciliary hairs on 



