424 



EMBRYOLOGY 



This points to the fact that even in Amphiura or its an- 

 cestors a metamorphosis took place, which, however, was 

 abandoned owing to a change in the mode of life. 



Echinoidea. — The larva resembles that of the Ophi- 

 uroidea, and, like it, is called a Pluteus. In it also the anal 

 area preponderates on the ventral surface. The ciliated 

 band is simple. A calcareous skeleton is found inside the 

 body and its appendages (Figs. 204 and 212, p. 440). 



The derivation of the Echinoid Pluteus from the funda- 

 mental form is nearly the same as in the Ophiurans, and is 

 explained by the diagrams of Fig. 203. The shape of the 

 different sea-urchin larvae is quite varied, according to the 

 greater or less development of the arms. The larvae of 



Fig. 203.— Evolution of the Echinoid Phi.teus from the fundamental form of the 

 Echinoderm larva (diagram after Joh. Muller, from Balfoue's Comparative Em- 

 bryology). For further particulars consult the explanation of Fig. 202. 



Echinus and Spata7igus may be distinguished as particularly 

 characteristic forms. On the anal area of the former, after 

 the development of all eight processes, the so-called ciliated 

 epaulettes make their appearance (Fig. 202 D). These are 

 two pairs of ciliated projections of the body, which lie on 

 either side immediately behind the ciliated band, but isolated 

 from it. According to A. Agassiz, they should be interpreted 

 as detached parts of the ciliated band. 



The larvae of Spatangus do not possess the ciliated epau- 

 lettes, but have three processes on the anal area (Fig. 203 



