GILLS NERVOUS SYSTEM 



353 



ou&r 



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, Digitibranchus 

 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 nii FlG - 18 9. Transverse section through 



rules ol priority, to be called, have Branchium sowerbyi. x 20. d.br, 

 not got gills, as is the case too Dorsal branch / a ; ^intestine ; , 



nerve-cord ; v.br, ventral branchia. 



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- 

 branclms 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. 11 2 A 



