THE GASTROPODA 87 
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- 
Fic. 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. /’, acrecbolic (=pleurembolic) 
introvert, formed by the snout of the proboscidiferous Gastropod. al, aliinentary canal; ¢, 
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 b. G, the acrecbolic snout of a proboscidiferous Gastropod, arrested 
short of complete eversion by the fibrous band b. 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 (Ff) a to d is inverted body surface. 
I, partial eversion of H. 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 
