272 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Fam. Lineidje. 



With head slits. 



Genera. Lineus. 



Euhorlasia. 



Micrura. 



Cerebratulus. 



Micrella. 

 The genera of the Lineidse set down above are exceed- 

 ingly difficult to define. The three genera Micrura, 

 Cerebratulus and Micrella (together with the exotic genus 

 Langia) agree with one another in the possession of a 

 slender tail filament at the posterior end of the body. 

 For this reason they have been grouped together as 

 Micrurse in opposition to the rest of the family, which are 

 known as the Amicrurse. It is very doubtful, however, 

 whether this caudal appendage is homologous in all the 

 instances in which it is found, for in some cases the anus 

 opens at its tip, whilst in others it opens at its base either 

 just above or just below it. It also presents other 

 anatomical differences in different species. 



The body form is regarded by some as affording a 

 character upon which to base generic distinctions. 

 Especially is such the case in Cerebratulus, of which genus 

 the species are often characterised by their breadth, due 

 chiefly to the sides of the animals being flattened out to 

 form a kind of fin known as the side folds. Gradations 

 between such a state and a more or less circular outline 

 in section are found, so that the absence of well-marked 

 side folds does not necessarily preclude a species from 

 being relegated to this genus. Cerebratulus is also sup- 

 posed to be characterised by a fine layer of diagonal 

 muscles just outside the circular layer. This, however, 

 is often absent. In fact at present the three genera, 

 Lineus, Cerebratulus and Micrura are exceedingly ill- 



