ECHINODERMATA 



425 



E), which are supported by calcareous rods, like the other 

 processes of the body. In the Plnteus of' Arbacia there are 

 only two processes on the anal area (Fig. 204), but they are 

 particularly long. Furthermore, in addition to the ordinary 

 Pluteus-arms, it possesses two pairs of auricular processes 

 (Fig. 204) which, like the arms, are surrounded by the 

 ciliated band (JoH. Muller, Fewkes). A pedicellaria of the 

 future sea-urchin can already be recognized on the anal area 

 of this larva. 



Fig. 20i.— Pluteus larva of Arbacia pustulosa (after Joh. Muller). P, pedi- 

 cellaria. The skeletal rods are dark. 



The skeletal parts are developed very early as products of the me- 

 senchyma (Selenka, No. 53 ; Ludwig, No. 34). There is first secreted 

 between two cells a calcareous concretion, which soon enlarges and 

 becomes triradiate. The skeletogenous cells then migrate along their 

 respective rays, gradually moving farther away, while they continue to 

 secrete calcareous salts. In this way finally arise the long skeletal rods, 

 which may be many times branched and perforated like a network 

 (Figs. 204 and 212, p. 440). 



The typical larval form may also be omitted in the sea- 

 urchins. Thus A. Agassiz (No. 4) describes a viviparous 



