88 THE ZOOLOGIST. 



are so similar in general appearance that they may readily be 

 confused. Thus H. dicksoni may easily be mistaken for H. 

 rhcetica. H. curiosa and H. lampas have much in common. 

 H. attenuata is easily distinguished from H. heterotropa by its 

 attenuated form. 



I have come to the conclusion that the conditions under 

 which these tiny annelids exist greatly affect some of their 

 characters. Under stimulating conditions not only do they 

 attain a larger size, but the setae frequently increase in number. 

 Hence there are two or three species which have polychaet 

 varieties, the number of setse reaching 8-12 per set in each 

 segment. Thus an individual with 50 segments would have no 

 fewer than 2000 setse, though not more than half an inch in length. 



It is now possible to present our facts in a systematic form, 

 and in preparing the following key I have followed the lines laid 

 down in my former paper (2), which is here extended to include 

 all the known British species. 



Synopsis of the Genus. 



i. Two pairs of sperrnathecae present. One species only, viz. 



Henlea puteana. 

 ii. One pair of sperrnathecae present. Varying much in character. 



1. Consisting of a simple duct. 



2. A duct with median bulb. 



3. A duct with ampulla attached to the intestine. 



4. Glands at the 4-5 opening. 

 We find further species with: — 



i. No oesophageal glands present ; these we name the Henlea- 



nella group, 

 ii. One pair of oesophageal glands — 



1. In segment 7. 



2. In segment 8. 



iii. Two pairs of oesophageal glands. One species only, viz. 

 H. ventriculosa. These may be regarded as the true 

 Henlea group. 

 Finally we have another clue in the septal glands, as follows : — 

 i. Two pairs of septals in H. mariona and H. fridericioides. 

 ii. Three pairs in all others, except 

 iii. Four pairs in Henlea trisetosa and H. quadrupla. 



The exceptions are always easy to find, but for the bulk a 

 key is required. 



Finally we note the species with nephridia-like glands, whose 

 character is not yet determined. 



