46 COMPARATIVE ANATOMY. 



The nervous collar, consisting of supra-cesophageal and sub- 

 cesophageal ganglia, connected by commissural strands, is seen 

 surrounding the oesophagus. From the sub-cesophageal ganglia 

 two nervous chords pass obliquely to a ganglion on the right, 

 (indicated by a black bristle), situated immediately in front of 

 the spermatheca : from this ganglion nerves are given off to the 

 genitals, liver, and branchiae. From the cephalic ganglia nerves 

 are seen passing to the tentacles. 



Order 2. Prosobranchiata. 



74. A Common Whelk (Buccinum undatum}. 



The proboscis has been extended to its full length, and a 

 blue rod inserted in the mouth. The foot and operculum are 

 in the positions which they would assume when the animal is 

 in motion. 



75. Another specimen, to shew the structure of the pro- 

 boscis, which has been laid open. A blue rod has been inserted 

 into the oesophagus. At the end of the proboscis is seen the 

 strap-like tongue, armed with a treble row of horny teeth. At 

 the end of the proboscis nearest to the head it is marked 

 horizontally with numerous striations, indicating the mode in 

 which it is folded upon itself within the body of the animal. 

 Its termination, which is not so marked, remains unfolded 

 when the organ is drawn in. The movements of the tongue 

 are effected by the longitudinal muscles seen beneath the oeso- 

 phagus. 



76. Another specimen, to shew the penis and the sperm- 

 duct. 



The penis has been protruded, and a portion of the shell 

 taken away so as to shew the sperm-duct, under which, where 

 it emerges from the body of the animal, a black bristle has 

 been placed. The penis is a very large, flattened organ, and 

 when laid open is found to be composed of interlacing muscular 

 fibres. The sperm-duct, having pursued a tortuous course 

 through the penis, terminates by opening externally at the 

 extremity of a projection near its blunt end. 



77. Another specimen, to shew the digestive tract. 



The whole of the mantle has been removed, the animal sus- 

 pended by its penis and proboscis, and the alimentary canal 



