ECHINODERMATA 417 



attaching itself. Furtliermoi'e, on the left side, between the 

 third and fourth ciliated bands, the water pore makes its 

 appearance as a clear spot on the yellowish-brown larva. 

 In addition to the systems of organs already considered, 

 the earliest fundaments of the skeleton can be recognized 

 (Fig. 197j. 



Holothurioidea. — The larvae of the Holothurioidea usu- 

 ally exhibit a typical form, which was designated by Joh. 

 MtJLLER as the Auricularia. Its derivation from the funda- 

 mental form of the Echinoderm larva is illustrated by the 

 following diagram^ (Fig. 198), in which the shaded part 

 represents the deep depression of the body, within which the 

 mouth-opening (m) lies. This part is surrounded by a band 

 of cilia, the transverse tracts of which, those lying in front of 

 the mouth and in front of the anus, have been distinguished 



an 



Fig. 198. — A to D, development of the Auricularia from the fundamental form of 

 the Echinoderm larva' (diagram after Joh. Muller, from Balfour's Comimrative 

 Embryology). The broad black line indicates the ciliated band, the shaded area 

 the depressed part of the surface, an, anus; in, mouth. 



from the tracts extending lengthwise on both sides, the 

 so-called longitudinal parts of the ciliated band (Figs. 198 

 A and B). The anus lies near the posterior pole of the larva. 

 In its further development the body of the larva, in front 

 as well as behind, becomes more hollowed out on both sides, 

 while the elevated parts of the ventral surface persist and 

 grow toward each other (Fig. 198 B and C). In this way 

 there results a larval form, on the ventral side of which an 



1 The manner of orientation chosen by Joh. Muller has been retained 

 in this and in the following diagrammatic figures (200, 202, and 203) for 

 practical reasons only. It would be better if they were placed with the 

 mouth upwards and the anus down, as has been done, for example, in 

 Figs. 199, 204, 209, and 211. 



K. H. E. E E 



