178 COMPARATIVE ANATOMY CHAP. 



of the ectodermal and the endodermal portions of the intestine are 

 difficult to determine. 



A. Buccal Cavity, Snout, Proboscis. 



The alimentary canal lias an oral aperture bordered by variously - shaped lips, 

 and in many Glossophora (in nearly all Gastropoda) leads into a vestibule or 

 anterior cavity roofed over by the lips and lined by a continuation of the outer wall 

 of the head. The dermal glands are not unfrequently (many Opisthobranchia and 

 a few Prosobranchia) more strongly developed on the lips as labial glands. In 

 many Gastropods, when the lips open, the mouth is able to seize and hold prey like 

 a sucker. 



Where the snout is short it is simply contractile (the Chitonidce, the Dioto- 

 cardia, most herbivorous Tcenioglossa, and many Pulmonata and Nudibranchia). 

 In this case the parts immediately surrounding the mouth are so strongly con- 

 tractile that when contraction takes place the mouth is drawn in somewhat so as 

 to lie at the base of a depression. An exaggeration of this arrangement, combined 

 with the prolongation of the snout, leads to the formation of the retractile or 

 proboscidal snout. The snout can in such cases be invaginated from its tip, i.e. 

 from the oral aperture into the cephalic cavity, the mouth then lying at the base of 

 the invagination (many Tectibranchia, Capulidce, Strombidce, Chcnopidtz, Calyptrceidce, 

 Cypraeidcc, Lamcllariidcc, Naticidcc, Scalaridoe, Solariidce). 



Finally, in many carnivorous Prosobranchia (Tritoniidce, Doliidcc, Cassidid" . 

 RaekifflotU, and a few Toxoglossa) a proboscis, often very long and enclosed in a 

 special proboscidal sheath, is developed (Figs. 71 and 152) ; this sheath lies in the 

 cavity of the head, which is often prolonged like a snout, and may even stretch back 

 into the body cavity. The oral aperture lies at the free anterior end of the 

 cylindrical proboscis, and we have to regard the proboscis with its sheath as a very 

 long snout, the base of which, however, is permanently invaginated into itself. In 

 this way the proximal portion of the snout forms the permanent proboscidal sheath, 

 while the distal portion with its terminal oral aperture forms the proboscis. Neither 

 of these portions can be invaginated or evaginated ; it is merely a zone lying 

 between them which takes part in the retraction of the proboscis into the body 

 cavity. This zone, when so invaginated, forms a temporary backward prolongation 

 of the proboscidal sheath, but when the proboscis is protruded forms the basal 

 portion of the latter. The permanent portion of the proboscidal sheath is connected 

 with the wall of the head by bands which make its evagination impossible, and the 

 inner ,wall of the permanent proboscis is connected by muscles or bands with the 

 O3sophagus lying within it, so that this portion of the organ cannot be invaginated ; 

 the oral aperture can thus never lie at the base of the proboscidal sheath. 



When the proboscis is retracted, there is therefore an aperture at the anterior 

 end of the snout or the head, which is not the oral aperture, but that of the 

 proboscidal sheath. When the proboscis is protruded, it projects beyond the 

 aperture of the sheath and carries at its point the oral aperture. 



The proboscis is retracted by means of muscles attached at the one end to the 

 body wall and at the other to its (invaginable) base. In its protrusion, a flow of 

 blood towards the snout probably plays the chief part, accompanied by contraction 

 of the circular muscles of the head and proboscis. 



The (carnivorous) Pteropoda gymnosomata also have a protrusible proboscis (Fig. 

 17, p. 11) provided with so-called buccal appendages. The same is present in the 

 allied Aplysiidw, but is weakly developed. The Thecosomata have no proboscis. 



The buccal cavity of Dentalium is noteworthy. It extends throughout the 



