ANATOMY AND OLASSiriCATION OP THE AEENIGOLIDiE. 549 



narrow spaces filled with blood. The histology of the cardiac 

 body closely resembles that of the same orgau in the dorsal 

 vessel of Terebellids^ as described by Picton. Each invagi- 

 nation has an outer endothelium, a muscular middle coat, and 

 an irregular central cavity in which the peritoneal chlorogo- 

 genous tissue forms an irregular network of granular, glan- 

 dular, and often vacuolated cells, sometimes exhibiting a more 

 regular arrangement approaching an epithelium. These 

 peritoneal cells are the essential part of the heart-body, and 

 the function of the latter may be considered to be at least 

 twofold, excretory and valvular. On the one hand it removes 

 waste matters from the blood of the heart, on the other it 

 serves to prevent the regurgitation of blood into the gastric 

 vessels during systole. 



VIII. With the exception of A. Claparedii all the species 

 of Arenicola possess longitudinal giant-fibres and seg- 

 mentally arranged "giant-cells'^ in the nerve-cord. The 

 giant-cells have been specially examined in A. Grubii. They 

 exhibit a complicated structure, and are enclosed in a sheath 

 of neuroglial fibrillse which penetrate into them for a short 

 distance. The protoplasm is histologically identical with that 

 of ganglion-cells, and in particular contains structures which 

 we believe are identical with Apathy's neuro-fibrillge. The 

 giant- cells are unipolar, and the process is free from chromo- 

 philous granules. This process we regard as an axon. It 

 contains the same kind of neuro-fibrillge which are found in 

 the body of the cell, but they are stouter. In the case of each 

 of the anterior and posterior three or four giaut-cells the 

 process enters a median giant-fibre. In the case of several 

 of the intermediate giant-cells the process enters one or both 

 lateral giant-fibres. In either case, however, before doing so 

 it gives off branches to the fibrous matter of the cord. The 

 giant-fibres have the following structure : — a central, partly 

 fibrillar, partly homogeneous substance, enclosed in a myelin 

 sheath, and this again in a neuroglial one. There is free 

 anastomosis between the median and lateral fibres, and 

 between the two latter inter se. Branches from the lateral, 



