DEVELOPMENT. 



137 



Later the anus shifts on to the ventral surface, and the cilia 

 become confined to definite tracts or bands. According to the 

 arrangement of these ciliated bands, two main types of larvae 

 can be distinguished. In one (A, Fig. 89) there is a single 

 longitudinal band of cilia 

 which passes across the ven- 

 tral surface immediately in 

 front of the mouth and in 

 front of the anus, and bounds 

 a depression on the ventral 

 surface into which the mouth 

 opens. This is the Aurieularia 

 type of larva which is found 

 in Holothurians ; a modifica-- 

 tion of it is found in Echinoids 

 and Ophiuroids, where it is 

 known as the Pluteus larva 

 (Fig. 94). In the other (B, 

 Fig. 89) there are two bands 



of cilia, the one preoral and encircling the preoral lobe, the 

 other longitudinal in appearance, but really postoral, forming 

 a complete circuit of the body between the mouth and the 

 anus ; this is the Bipinnaria type of larva and is characteristic 

 of Asteroids. The larvae of both these types are externally 

 bilaterally symmetrical and have been so since the ventral sur- 

 face became distinct. They remain symmetrical externally for 

 some time longer, but at about this stage, or in some cases even 



Fio 



. A, the larva of a Holothurian 



(Auricularia type) ; B of an Asteroid (Bipin- 

 naria type), both seen from the left side 

 (from Balfour). a anus ; m mouth ; st 

 stomach ; l.c longitudinal ciliated band ; pr.c 

 preoral ciliated band. 



FIG. 90. A series of diagrams representing the evolution of an auricularia larva from the 

 simplest Echinoderm larval form, ventral view (from Balfour). The black line represents 

 the ciliated band. The shaded part is the oral side of the ring (oral depression, see Fig..89), 

 the clear part the aboral side of the larva, m mouth ; an anus. 



before this stage is reached, their internal structure begins to 

 show traces of that asymmetry which is so characteristic of the 

 later larvae and eventually leads to the establishment of the 

 radial symmetry of the adult. Before, however, considering 



