METAZOA ECHINODERMA. 179 



Agassiz l has shown that the young of all other Echini 

 have the general characteristics of these primitive forms. 

 They all agree in having a small number of plates in the 

 corona, slight separation of the actinal and abactinal 

 areas from the corona proper, nearly vertical rows of 

 paired pores, and a few spines of disproportionate length. 

 Having this common origin we shall see what variations 

 arise in the adults of a number of species. 



In Arbacia pustulosa Gray (No. 324), the actinal area 

 is large and the ambulacra are broad at the starting 

 point, growing narrower as they reach the edge or ambi- 

 tus. The interambulacra, on the other hand, are narrow 

 at the ventral border and broader towards the ambitus. 

 The pores preserve their primitive character, being in sim- 

 ple vertical rows. The anal disc in this species consists 

 of five plates ; around these is the ring of five genital 

 plates which are developed after the anal disc. The ocu- 

 lars are crowded outside of this ring and fit into the places 

 left by the outer angles of the genitals. This genus is 

 interesting for the fact that its spines never become artic- 

 ulated but remain in the more primitive condition of the 

 unjointed spines of the starfish. 



In Diadema setosum Gray (Nos. 325, 326), the nearly 

 vertical row of pores in the narrow ambulacra becomes 

 changed during the growth of the animal into nearly ver- 

 tical arcs of three or four pairs of pores. The corona is 

 thin with broad interambulacra. The actinal area is 

 membranous with well developed teeth. The anal disc 

 (No. 326) is also membranous and flexible, with the 

 anus raised on a tube near the center. According to 

 A. Agassiz 2 this anal tube is as prominent in the young 

 as the anal tube of some species of Comatulae. Three of 



1 Palaeontological and Embryological Development, Proc. Amer. 

 Assoc. Adv. Sci., XXIX, 1880, p. 389; also consult review of the 

 same by E. D. Cope, Amer. Nat., Oct., r88o, p. 725. 



2 Rev. of Echin., Mem. Mus. Comp. Zool., Ill, 1872, p. 276. 



