ECHINODERMATA 



423 



stage of the larva two triradiate calcareous bodies are 

 secreted by tlie mesenchyma cells in the vicinity of the 

 blastopore. These soon elongate owing to the activity of 

 the mesenchyma cells. They increase considerably in 

 volume, branch, and send out rod-like processes into the 

 arms as soon as the latter are developed. The calcareous 

 i-ods fuse at the posterior end of the larva, and appear to be 

 united here by a kind of ring having a transverse position 

 (Fig. 211, p. 438). In this way there arises an excellent 

 supporting apparatus for the larva and its appendages. 



Fift. 202.—^ to D, evolution of the Oplduvan Pluteus from the fundamental form 

 of the Echinoderm larva (diagram after Joh. Mijllkk, from Balpoub's Comjiarative 

 Emhi-yology). The Ijroad black line indicates the ciliated band, the shaded area 

 the depressed part of the external surface. In regard to the orientation, what is 

 said on p. 417 (footnote) applies here also, an, anus ; m, mouth. The other letters 

 refer to the nomenclature of the appendages, which is not further considered here. 



AmpJiiura squamata develops without any real meta- 

 morphosis. Amphiura is viviparous. The earliest develop- 

 mental processes are nearly the same as those that we have 

 already learned about. There arises an oval embryo, which 

 assumes a bilaterally symmetrical shape, but which does 

 not develop into a ciliated larva, passing, on the contrary, 

 directly into the five-rayed star. The young, even at the 

 time when they come into the world, exhibit the organization 

 of the parent. It is interesting, however, that, despite this, 

 the larval skeleton of the Pluteus is begun in the embryos. 



