30 



1| inch long in the adult, is thin walled, made up of 

 muscular fibres and connective tissue, and is connected by 

 long strands of muscle with the walls of the body cavity. 

 Some of these strands pass backwards for some distance. 

 Now this region can be everted in such a way that whilst 

 forming part of the proboscis sheath in the retracted 

 condition (fig. a), it forms the base of the proboscis 

 in the extended position (fig. b). Thus the protrusion 

 of the proboscis is not due to muscular action of this 

 structure itself but to the unfolding of the hinder part of 

 its sheath. The proboscis is nothing but a further con- 

 tinuation of the body wall, and the true mouth lies as 

 mentioned above at its distal end (figs, a and B, M .). 



The cavity (part of the external world) between the 

 proboscis and its sheath (Text-fig. 4, Rhyn., and PL II, 

 fig. 11, Rhyn.) is known as the Kliynchodaeum, and 

 the opening of the latter, or the false mouth, is the 

 Rhynchostome (Rst.). Such a proboscis is termed a 

 pleur embolic proboscis (Lankester*), from the fact that 

 when withdrawn it is the base that is pulled and 

 disappears first. The other and opposite type is the 

 pleur ekbolic, met with in the Cypraeidae, etc. The 

 proboscis of Buccinum was known to the ancients, and 

 both Aristotle t and Pliny % refer to it. Cuvier was the 

 first, however (1817) § , who described it with accuracy and 

 detail. 



Pharynx The mouth (Text-fig. 4, M., and fig. 11) 



opens into a muscular pharynx (PL II, fig. 12, Ph.), the 

 walls of which are attached all round to the proboscis 

 walls by radiating muscles. Into the floor of the 



* Lankester. Art. Mollusca, Encyclop. Britannica, 9th edit., 

 Vol. XVI, 1883. 



f Aristoteles. De Animal, hist., Lib. IV, cap. 4, §§ 7, 8, 9. 

 } Plinius. Hist. Nat., L. XI, c. 37. 



§ Cuvier. Mem. pour servir a l'hist. et a l'anatomie des 

 Mollusques. 



