III. CRIXOIDEA 



299 



the ambulacral grooves are closed, and the ambulacra, which lack 

 sucking discs, are tactile, locomotion being effected by the snake-like motion 

 of the arms. The madreporite is on 

 the ventral surface. Also on the ven- 

 tral surface are five slits which con- 

 nect with as many burscc, thin-walled 

 respiratory sacs into which the sexua! 

 organs open. The gonads are at- 

 tached to a genital rhachis which coils 

 through the disc. 



In many brittle stars, especially in 

 young specimens, there is a kind of 

 asexual generation (schizogony), the 

 animal dividing through the disc, the 

 halves regenerating the missing parts. 

 OPHIURID.^, arms unbranched (Ophio- 



pholis* (fig. 292), Ophioglypha* Atnphhtra*); EURYALID.E, the arms branched 

 (Astrophyton,* fig. 293). 



FIG. 291. Section of Gphiuroid 

 arm (orig.). a, ambulacrum; b, blood- 

 vessel; c, ccelom; m, muscles of arm; 

 n, nerve; r, radial water tube; v, 'ver- 

 tebra' (coalesced ambulacral plates). 



FIG. 292. Ophiopholis aculeata* 

 (from Morse). 



FIG. 293. Astrophyton arborescens, 

 basket fish (from Ludwig). 



Class III. Crinoidea (Pelmatozoa). 



The crinoids or sea lilies are on the road to extinction. In early times, 



j 



especially in the paleozoic, they were very abundant, but to-day there are 

 but few species, these mostly restricted to the greater depths of the ocean, 

 only the Comatulidse occurring near the shore. The crinoids are attached 

 to the sea bottom by a long stalk (fig. 294), composed of cylindrical discs 

 which often bear five rows of outgrowths, the cirri. The young Coma- 

 tulida? (fig. 295) are similarly attached, having a Pcntacrhuis stage, but 

 later they separate and live a free life, a proof that the attached condition 

 was primitive. When the separation takes place, one joint of ilu- sulk 



