THE GASTROPODA 



tegumentary ingrowth, which in this manner gives rise to a 

 proboscis. When this extensible proboscis is evaginated the 

 oesophagus forms its interior lining ; when it is invaginated the 

 oesophagus forms its posterior continuation. Such is the pleurec- 



FIG. 72. 



Diagrams explanatory of the nature of so-called proboscides or "introverts." A, simple 

 introvert completely introverted. B, the same, partially everted by eversion of the sides, a.s in 

 i lie Ni'innrtine proboscis and Gastropod eye-tentacle =pleurecbolic. C, the same, fully everted. 

 1>. /,. a similar simple introvert in course of eversion by the forward movement, not of its sides, 

 butof its apt-x, as in the proboscidean Rh,ibdocoels = acrecbolic. F, acrecbolic( = pleurembolic) 

 introvert, formed by the snout of the proboscidiferous Gastropod, al, alimentary canal ; d, 

 the true month. The introvert is not a simple one with complete range both in eversion and 

 introversion, but is arrested in introversion by the fibrous bands at c, and similarly in eversion 

 by the librous bands at l>. G, the acrecbolic snout of a proboscidiferous Gastropod, arrested 

 short of complete eversion by the fibrous band b. 11, the acrembolic (=pleurecbolic) pharynx 

 of a Chaetopod fully introverted, a!, alimentary canal ; at <f, the jaws ; at a, the mouth ; 

 therefore a to d is stomodacum, whereas in the Gastropod (F) n to d is inverted body surface. 

 I, partial eversion of //. A", complete eversion of //. (After Lankestcr.) 



bolic proboscis of Kay Lankester (Fig. 72) found in the Cypraeidae, 

 Naticidae, Lamellariidae, Scalariidae, Permetus, the Capulidae, 

 Calyptraeidae, Strombidae, and Chenopodidae among the Strep- 

 toneura, and in some Opisthobranchs, viz. Doridium, the Pleuro- 

 branchidae, Aplysia, the Gymnosomata, and the Doridopsidae. In 

 other cases the proboscis cannot be wholly retracted, and then the 



