defined. Many anatomical differences are found in the 

 family, amongst which may more particularly be men- 

 tioned the following:- — 



(a) A diagonal muscle layer, neurochord cells, eyes, and 



frontal organ may be either present or absent. 

 (J)) A Avoll-marked cephalic vascular head loop may be 

 present, or the cephalic vessels may form an 

 anastomosing network. 

 (e) The excretory system shews great variations in its 

 backward extent ; the position of the tubules 

 may be dorsal or ventral, or both ; they ma}' 

 reach forward to the cerebral organ, or may com- 

 mence some way behind it. Also some species 

 possess a number of ducts whilst only one pair is 

 present in others. 

 In the majority of the Lineida^, and indeed in nian\ 

 Jh'itish forms, we are as yet in ignorance with regard \(> 

 many of these j^oints, and until they have been deter- 

 mined it is useless to attempt to place the classification of 

 the family upon a more satisfactory basis. 



