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- 



L 



^ 



f\ 



fll 



\ 



al- 



Fio. 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, as in 

 the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic. C, the same, fully everted. 

 D, E, a similar simple introvert in course of eversion by the forward movement, not of its sides, 

 but of its apex, as in the proboscidean Rhabdocoels = acrecbolic. F, acrecbolic ( = pleurembolic) 

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

 the true mouth. 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 fibrous bands at I). G, the acrecbolic snout of a proboscidiferous Gastropod, arrested 

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

 of a Chaetopod fully introverted, al, alimentary canal ; at d, the jaws ; at a, the mouth ; 

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

 I, partial eversion of //. K, complete eversion of H. (After Lankester.) 



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

 Naticidae, Lamellariidae, Scalariidae, Vermetus, 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 



