M ETAZOA ECHINODERMA. 1 65 



small underbasals (fig. i) and five basals (shaded in 

 fig. i). Outside of these are five radials. Regular 

 plates extend outward to the tip of the arms, where are 

 found the terminal or ocular plates. This definite ar- 

 rangement of plates so finely illustrated by both the 

 larval and adult Zoroaster, occurs in some of the larvae 

 of the more specialized Asteroidea, but is soon lost in the 

 process of development. 



While these characters are all of a primitive nature, 

 Zoroaster possesses other peculiarities which place it 

 near the more specialized genus Asterias. The disc is 

 small and the arms long and tapering (fig. 2, showing 

 spines but not plates). The tube feet have sucking discs, 

 and, unlike the genera already described, there are four 

 rows of these locomotive organs (fig. 3). 



One of the commonest starfishes on the New England 

 coast is Asterias forbesi Verr. (Nos. 302-308). This ani- 

 mal passes through an indirect development with a 

 marked and peculiar metamorphosis. 



Like most starfishes when young, the larva or brachio- 

 laria, as it is called, is bilateral with four arms on either 

 side, so that the marked radial symmetry which appears 

 later is a secondary and not a primitive character. 



The aboral side is raised and more or less dome- 

 shaped. The spines are in regular rows, according to A. 

 Agassiz, and the plates remind one of those of Crinoids. 

 Underbasal plates have been found in two species of 

 Asterias (A. rubens and A. gtacialis 1 }. The basals and 

 radials appear in the very young larva and are homolo- 

 gous with the same plates in the Crinoid ; but as develop- 

 ment goes on, it becomes impossible to trace them. By 

 many authors the terminals or ocular plates of starfishes, 

 Ophiurans, and sea urchins have been considered as 

 homologous with the radials of Crinoids ; but it has been 

 shown 2 that the radials in starfishes are developed be- 



1 Sladen, Quart. Journ. Micr. Sci., XXIV, 1884, p. 34. 



2 Sladen, ibid., p. 29. 



