GILLS NERVOUS SYSTEM 



353 



instances ; but as this worm was only known from a fragment, 



and as the description was not by 



any means full, it was not thoroughly 



believed in ; it was surmised that it 



might be a member of some marine 



genus, perhaps of the Capitellidae. 



Oddly enough, the same worm was 



independently described by a 



different name, Digitibranclius 



niloticus, a few years later by 



Levinsen. Quite recently Michael- 



sen has found by a reference to 



the original types that this worm 



is really gilled, and that it is speci- 



fically identical with a worm which 



had been given a totally different 



name, viz. Siphonogaster. The fact 



that the gills of the latter had 



been overlooked was readily ex- 



plained by the circumstance that 



they are retractile, and not merely 



contractile. But all the species of 



the genus Siphonogaster, or Alma, 



as it ought really, following the 



J> , 



rules of priority, to be called, have 

 not got gills, as is the case too 



' 



with the genus Hesperodrilus. The 



gills of Alma are branched, and there is therefore no longer any 



justification whatever for defining the Oligochaeta as a group of 



Annelids without gills. The simple gill-like processes of Chaeto- 



branchus might have been held to be not accurately compar- 



able to the more complex structures which we find in the marine 



worms. 



Nervous System. The central nervous system of the 

 Oligochaeta is very uniform in its structure in the entire group. 

 The only family which is at all anomalous is that of the 

 Aphaneura. In Aeolosoma there appears to be only a pair of 

 cerebral ganglia, which retain the primitive position of these 

 organs in being still in direct connexion with the epidermis. 

 In all other Oligochaeta there are a pair of cerebral ganglia, 



VOL. II 2 A 



ufcr- 







FIG. 189. Transverse section through 



Branchium smcerbyi. x 20. rf.fr/-, 

 Dorsal b nchia; <, intertine ; , 



uerve-cord ; v.br, ventral bronchia. 



