METAZOA ECHINODERMA. 173 



from a phylogenetic point of view. As Goniocidaris 

 grows older, two rows of interambulacral plates arise from 

 the single plate (fig. i, i, 2 ; also fig. 2) so that there are 

 two rows of ambulacral plates alternating with two rows 

 of interambulacral plates. The ambulacral plates become 

 differentiated and are lower than the pentagonal interam- 

 bulacral plates, while each plate has two pores. 



The adult Goniocidaris never goes beyond the stage rep- 

 resented by the two rowed ambulacra and interambulacra. 



Correlated with this simplicity of external structure we 

 have a primitive mode of development. In other words, 

 Goniocidaris develops from the egg without passing 

 through a metamorphosis. 



We have already seen that while a few starfishes develop 

 in a primitive way, most of them pass through a complex 

 metamorphosis. The early stages "of Echinoids that under- 

 go such a transformation are similar to those of starfishes. 

 The embryo or pluteus has eight arms and is bilaterally 

 symmetrical. The metamorphosis of the Echinoid, how- 

 ever, is accomplished very rapidly. "In less than an 

 hour," according to Bury, 1 "a perfect Pluteus is trans- 

 formed into a small, rounded Echinoid in which radiate 

 symmetry entirely replaces the bilateral symmetry of the 

 larva." 



The internal structure of the young Goniocidaris is 

 primitive. For a time the intestine is a closed tube, there 

 being no mouth nor anus. During this period the animal 

 takes no food, and moves about by five provisional tube 

 feet. It is later that the eating apparatus is developed 

 which causes a modification of the oral area and a resorp- 

 tion of some of the plates of the corona ; finally the intes- 

 tine breaks through the anal disc. 



The genus Cidaris (No. 316, C. thouarsi Val., with 

 spines; No. 317, without spines) when young possesses 

 the circle of ventral plates entire, and also the primitive 



1 Quart. Journ. Micr. Soc., XXXVIII, 1895, p. 77. 



