ANNELIDA 271 



of the Trochophore larva, for the anterior bell- shaped part in 

 the majority of cases is not retained unaltered for so long 

 a time. Generally also it does not surpass the trunk so 

 considerably in size, and it soon comes to be even smaller 

 than the trunk. Since in many forms the typical shape of 

 the Trochophore is not so strongly expressed, and, on the 

 other hand, the segmentation of the body of the worm makes 

 its appearance at an early stage, many deviations from the 

 shape of the Trochophore are realized. The larvae of 

 Annelids are very variously shaped, for some of them, owing 

 to the early appearance of the segmentation, are found in 

 phylogenetically younger stages than the Trochophore^ and 

 others, although they stand at the same level with it, 

 may be modified by the occurrence of various kinds of 

 locomotor organs and by other external changes in form. 

 The principal difference in the larvce consists in the presence 

 or absence of segmentation of the entire larva, not includ- 

 ing that of the trunk part, which is acquired only during 

 the metamorphosis. To be sure, this difference should not 

 be overrated, for the segmented forms likewise must pass 

 ontogenetically through an unsegmented stage. The Annelid 

 larvae have usually been distinguished according to the dis- 

 tribution of their cilia : as Atrochse when a ciliated band is 

 lacking ; Monotrochae with a preoral band of cilia, to which, 

 as in the Telotrochae, there may be added a post-oral band 

 lying directly behind the mouth ; Telotrochse with an anterior 

 and posterior (perianal) band of cilia ; Mesotrochse, in which 

 the ciliated band is situated in the middle of the body ; and, 

 finally, Polytrochse, which possess a greater or smaller num- 

 ber of ciliated bands, and as a result of this exhibit at an 

 early stage a segmentation of the body. The ciliated bands 

 of the Polytrochas may form either closed rings, or only half- 

 rings. In the latter case, according to their position on the 

 dorsal or ventral surface, Nototrochas and Gastrotrochse are 

 in turn distinguished. They are called Amphitrochse when 

 ventral and dorsal half-rings alternate with one another. 

 This classification has been made use of by different investi- 

 gators for distinguishing the larvae. However, ClaparIide 

 and Metschnikoff themselves, to whom we owe the most 



