50 COMPARATIVE ANATOMY. 



81. A Roman Snail (Helix pomatia), dissected to shew its 

 nervous system. 



The viscera have been removed, with the exception of the 

 buccal mass, oesophagus, and portions of the generative system. 



The oesophagus has been drawn through the nerve-collar 

 and pinned in front of the animal's head. 



As in other Gasteropoda the central nervous system forms a 

 collar encircling the oesophagus. The supra-cesophageal part of 

 the collar is composed of the two cephalic ganglia and the broad 

 commissural band connecting them. The sub-cesophageal gan- 

 glia, which are connected with the former on each side by a 

 double nervous cord, consist of a lower or pedal, and an upper 

 or parieto-splanchnic element. From the former of these ele- 

 ments nerves may be seen passing to the muscular foot, while 

 from the parieto-splanchnie portion three large nerves pass off 

 to supply the parietes. Of these, two pass to the right side, and 

 one, which bifurcates, to the left. Arising from the same source 

 is the great splanchnic nea-ve. This nerve proceeds posteriorly, 

 in close proximity with the anterior aorta, as far as the al- 

 buminiparous gland, at which point it divides into two branches, 

 one of which goes ,to the liver, and the other to the herma- 

 phrodite gland. The anterior aorta passes forwards through a 

 foramen between the parieto-splanchnic and pedal ganglia to 

 supply the anterior part of the body. This fact, together with 

 the distribution of the nerves of the two ganglia respectively, 

 enables these two elements of the sub-cesophageal mass to be 

 differentiated. 



From the cephalic ganglia nerves, under which a slip of blue 

 paper has been placed, may be seen passing forwards to supply 

 the superior or eye-bearing tentacles, together with the upper 

 lip and adjacent sensitive surface. 



From each cephalic ganglion a nerve passes to join a small 

 stomato-gastric ganglion, situated on either side of the point at 

 which the oesophagus joins the buccal mass. The two stomato- 

 gastric ganglia are connected by a commissural strand, and from 

 each nerves are given off to supply the buccal mass, salivary 

 glands, and oesophagus. 



82. A Grey Slug (Limax cinereus), dissected to shew the 

 general distribution of its organs. 



