PILIDIUM-LARVA OF NEMERTES. 



631 



type is presented by the Larva which has received the name 

 Pilidium (Fig. 329) ; its shape being that of a helmet, the plume 

 of which is replaced by a single long bristle-like appendage that 



Fig. 329. 



Pylidium gyrans : — A, young, showing at a the Alimentary 

 canal, and at b the rudiment of the Nemertid ;— b, more advanced 

 stage of the same ; — c, newly-freed Nemertid. 



is in continual motion, its point moving round and round in a 

 circle. This curious organism, first noticed by Muller, has been 

 since ascertained to be the Larva of the well-known Nemertes, a 

 Turbellarian worm of enormous length, which is commonly found 

 entwining itself among the roots of Algae. * 



* See especially Leuckart and Parenstecher's ' Untersuchungen iibsr 

 niedere Seethiere,' in "Muller's Archiv.," 1853, p. 569. The Author has 

 frequently met with Pilidium in Lamlash Bay. 



