638 ECHINODEEMATA 



ORDER 2. EURYALAE.* 



Brittle-stars in which the arms are often branched and can be rolled 

 in towards the mouth; axial plates of the arms double, the other plates 

 rudimentary or wanting, and the surface of the body covered by a soft 



skin: 3 families. 



FAMILY ASTEOPHYTIDAE. 



Girdles of minute hooks on the arms, at least near the tips; teeth 

 and the mouth and teeth papillae spiniform and indistinguishable from 

 one another: about 20 genera. 



GORGONOCEFHALUS Leach. Disc circular or pentagonal, with radial 

 shields on the aboral surface; each arm branches at its base, and from 

 11 to 15 times in addition: 8 species. 



G. agassizi (Stimpson). Basket-fish (Fig. 986). Disc 4 to 8 cm. 

 in diameter; length of arm 14 to 28 cm.: from Nantucket northwards, 

 from extreme low-water mark to 800 fathoms; common towards the 

 north. 



CLASS 4. ECHINOIDEA.f 



Sea-urchins. Echinoderms (Fig. 965), in which the five radii are 

 not elongated so as to form arms but are approximately of the same 

 length as the interradii, and the main body axis is short, so that the body 

 is subglobular in form or disc-shaped. The calcareous plates are exten- 

 sively developed and, except in the Echinothuriidae, fit closely together, 

 forming a case, the "test" (Fig. 987), which encloses a spacious body 

 cavity. The oral surface, with the mouth usually in its center, is more 

 flattened than the aboral, and is directed downwards; on it the animal 

 moves 'about. Surrounding the mouth is a circular, flexible area called 

 the peristome (Fig. 966, B, 3), and projecting from the mouth in most 

 sea-urchins are five calcareous teeth (Fig. 988). At the edge of the 

 peristome are, in most species, five pairs of branched outgrowths called 

 the gills (Fig. 966, B, 13). In the middle of the aboral surface is either 

 the anus surrounded by a system of plates called the periproct (Fig. 987) 

 or the madreporite (Fig. 990), the anus in the latter case being in an 

 interradius at or near the edge of the test. 



The ambulacral pores, through which the ambulacral appendages 

 extend from body cavity to the outside, are, with rare exceptions, present 

 only in the radii, four rows of pores appearing in each radius. In cer- 



* See "Ueber Japanische und andere Euryalae," by L. Doderleln, Abb. 5, K. 

 Bayer, Akad. Wiss., Math. Pbys., Kl. 2, Supp. 



t See "A Revision of the Echini," by A. Agassiz, 111. Cat. Mus. Comp. Zool., No. 7, 

 1872-1874. "Hawaiian and Other Pacific Echini," by A, Agassiz and H. L. Clark, 

 Mem. Mus. Comp. Zool., Vol. 34, 1907. "Comp. Morphology of the Echini," by R. T. 

 Jackson, Mem. Bost. Soc. Nat. Hist. 1911. "The Genera of Recent Clypeastroids," 

 by H. I/. Clark, Ann. Am. Mag. Nat. hist., Ser. 8, Vol. 7, 1911. 



