226 



COMPARATIVE ANATOMY 



CHAP. 



ventral portion of the oesophageal ring, and in some cases two lateral 

 swellings can be made out. 



From the supra-oesophageal ganglion two nerves run into the arms; 



from the infra- 

 oesophageal gan- 

 glion nerves run 

 also to the arms, 

 to the mantle, and 

 to the opening and 

 closing muscles of 

 the shell. The 

 arm nerves are 

 connected with a 

 plexus of gangli- 

 onic cells and 

 { nerve fibres, which 

 spread out in the 

 supporting sub- 

 stance of the arm 

 walls close under 

 the epithelium. 

 Each of the man- 

 tle nerves divides 

 into a dorsal 

 branch for the 

 dorsal fold of the 

 mantle, and a 

 ventral branch for 

 the ventral fold. 

 They are again 

 richly branched, 

 and their branches 

 anastomose form- 

 ing a plexus. The nervous system of the Bmchiopocla lies under the 

 integument. 



In the Bryozoa, only the supra-cesophageal ganglion of the oesoph- 

 ageal ring is retained ; it lies as a generally inconsiderable mass (occa- 

 sionally with thickened lateral parts) dorsally over the fore-gut, between 

 mouth and anus, under the integument. From this ganglion nerves 

 run chiefly into the tentacles, and further to the two ciliated pits, 

 when such are present. 



Rotatoria. — Over the oesophagus there lies a ganglion (supra-oesoph- 

 ageal ganglion) which sends off nerves to the wheel organ, the cutaneous 

 organs of touch, and the muscles. It lies under the integument. 



In Dinophilus also, in front of and above the mouth (in the pro- 

 stomium), is found a mass of fibres surrounded by ganglionic cells, which 

 represents the supra-oesophageal ganglion. In Dinophilus gigas two longi- 



FiG. 150. — Preparation of Terebratula vitrea for demonstration 

 of the nervous system, tlie ovaries, and the nephridia, after van 

 Bemmelen. Anterior body wall after removal of the arm apparatus, 

 the enteric canal, and tlie closing muscles, spread out and seen from 

 within. Above lies a part of the dorsal body wall ; below, a part of the 

 ventral body wall, g, Brain ; sc, oesophageal commissures ; usg, infra- 

 <:esophageal ganglion ; saji, arm nerve proceeding from the supra-oeso- 

 phageal ganglion ; ian, ditto from infra-resophageal ganglion ; ces, (Eso- 

 phagus ; dmn, dorsal mantle nerves ; lin, nerves proceeding baclcwards 

 from infra-oesophageal ganglion ; n, nephridia (oviducts) ; iit^ nephridial 

 funnel ; gf, genital folds ; ov, ovaries ; dm, dorsal mesentery ; vm, ven- 

 tral mesentery. 



