196 Studies in Enchytraeid Worms 



No fewer than 11 genera of Enchytraeids are now known to occur 

 in Great Britain. Taken alphabetically they stand as follows : Achaeta, 

 Bryodrilus, BucJtJiohia, Chamaedrilus, Enchytraeus, Fridericia, Grania, 

 Henlea, Lumhrieillus, Marionina and Mesevichytraeus. Of these, Henlea 

 is the first in order of treatment by Michaelsen (6), and a typical British 

 species of this genus has been chosen as the first representative of the 

 family for treatment in this series of studies. 



The genus Henlea was established by Michaelsen (5) in 1889, six 

 species being then recognized. Though the number was so limited, 

 the characters were somewhat heterogeneous. The main features were 

 as follows: Setae not of uniform size or arrangement. Head pore 

 between the prostomium and peristomium or first body ring (usually 

 represented by the symbol 0/1). No dorsal pores. Blood colourless. 

 Nephridia with the duct arising near the septum. Dorsal vessel arising 

 in front of the clitellum. Oesophagus sharply marked off from the 

 intestine. 



All the characters here enumerated, however, may be found in 

 one or other of the allied genera. In 1895 Beddard (1) discussed the 

 definition, which was five years later further extended and modified by 

 Michaelsen (6), who at this time admitted only five species as beyond 

 dispute. He drew attention to the following points: Coelomic cor- 

 puscles (Lymphkorper, lymphocytes) of one form only, large, mostly 

 discus-shaped, seldom elliptical, darkly granulated. The oesophagus 

 merging suddenly in the intestine in the 7th, 8th or 9th segments. The 

 origin of the dorsal blood vessel anteclitellian in segment 8 or 9, sperma- 

 thecae lacking diverticula, but communicating with the oesophagus. 



In the systematic arrangement the presence or absence of oesophageal 

 glands (Darmtaschen, intestinal diverticula) finds place. The recent 

 discovery of many new species both in Great Britain and abroad, 

 however, shows that not one of the characters in the new definition 

 is strictly generic. We meet with species whose coelomic corpuscles 

 differ from those of the type, others in which the oesophagus does 

 not merge suddenly but gradually into the intestine. In a few instances 

 the spermathecae possess diverticula, and are merely attached to, but 

 do not communicate with, the intestine. Finally, we have what seemed 

 to be the most distinctive characters swept away ; for certain species have 

 been discovered whose dorsal vessel does not originate in front of the cli- 

 fcellum, and whose intestine is destitute of special glands (Darmtaschen). 



Hence a new definition became necessary. There are many points 

 which would serve the systematist in his endeavours to distinguish 



