i 9 4 SCIENCE PROGRESS. 



acterised species. And this estimate does not take into 

 consideration subspecies or well marked varieties, and pays 

 no attention to " species incertae ". Of aquatic Oligochceta 

 150 is about the number of known species ; but this 

 group is decidedly less known than the former. As with 

 other groups of animals this great increase in the number 

 of known species has added to our knowledge of anatomical 

 fact, but rendered harder the formation of classificatory 

 schemes. No indistinctness, however, has arisen to blur 

 the perfectly sharp outlines of the group Oligochaeta, no 

 ''intermediate" forms have been discovered whose relega- 

 tion to the group is a matter of uncertainty or convenience. 

 At the same time a few approximations in structure to the 

 leeches on the one hand, and to the Polychseta on the 

 other have been discovered ; but these are in no case of 

 first-rate importance. Perhaps the most remarkable is the 

 description of the gills of the African genus Alma. This 

 worm was originally described under that name by Grube 

 in 1855. Thirty-four years later Levinsen, apparently in 

 ignorance of Grube's paper, named a fragment of what was 

 obviously the same worm Digitibranckus, and described in 

 the same paper Siphonogaster, an Annelid characterised by 

 a pair of long processes an inch or so in length, and of a 

 spatula-like form arising from the eighteenth segment. 

 These have been subsequently shown to be processes con- 

 taining the outer section of the sperm duct which opens 

 near to the extremity. Michaelsen showed that all these 

 three worms are identical, and has thus been able to put 

 beyond question the existence of a true earthworm l with 

 branched retractile gills on the posterior segments of the 

 body. It was not by any means clear from the earlier 

 descriptions that the gilled worm was not a Polychaet. 

 Among the lower aquatic Oligochsetes there are at least 

 three gilled forms, apart from Dero which has a circlet 

 of ciliated processes, with vascular twigs lying round the 

 anus. These forms are Chcstobranckus of Bourne, and 

 Branchiura and Hesperodrilus branchiatus of myself. In 



1 Structurally ; in habit it is aquatic. 



