LARVAL FORMS. 



305 



without any great breaks in it, as upon the existence of characters 

 common to the whole of them. It is also 

 worth noting that most of the characters 

 which have been enumerated as common to 

 the whole of these larvae are not such second- 

 ary characters as (in accordance with the 

 considerations used above) might be expected 

 to arise from the fact of their being sub- 

 jected to nearly similar conditions of life. 

 Their transparency is, no doubt, such a 

 secondary character, and it is not impossible 

 that the existence of ciliated bands may be 

 so also ; but it is quite possible that if, as 

 I suppose, these larvae reproduce the cha- 

 racters of some ancestral form, this form may 

 have existed at a time when all marine 

 animals were free-swimming, and that it 

 may, therefore, have been provided with at 

 least one ciliated band. 



The detailed consideration of the charac- 

 ters of these larvae, given below, supports 

 this view. 



This great class of larvae may, as already 

 stated, be divided into a series of minor subdivisions 

 divisions are the following : 



1. The Pilidium Group. — This group is characterised by the 

 mouth being situated nearly in the centre of the ventral surface, and 

 by the absence of an anus. It includes the Pilidium of the Nemer- 



FiG. 220. Labva of Ak- 

 oiopE. (From Gegenbaur ; 

 after Kowalevsky.) 



m. mantle ; b, setae ; 

 d. archenteron. 



These sub- 



FiG. 221. Two BTAGES IN THE DEVELOPMENT OF PiLiDiuM. (After Metschnikoff.) 

 ae. archenteron; oe. oesophagus; st. stomach; am. amnion; -pr.d. prostomial 

 disc ; po.d. metastomial disc ; c.s. cephalic sack (lateral pit). 



li. E. II. 20 



