338 



ECIITNODERMA. 



face. Also on the ventral surface are five slits wliicli connect 

 with as many bursaj, thin-walled respiratory sacs into which the 

 sexual organs open. 



In many brittle stars (Opldocnida, Ophiotlielia, OpJiiocoma), especially 

 in young specimens, there is a kind of asexual generation (schizogony), the 

 animal dividing through the disc, the lialves regenerating the missing parts. 

 The classificatioa is based largely on small details. In the majority the 

 arms are unbranched {Opldojjliolis * (fig. 318), OpMoglypJia* Amphiura *), 

 but in the EurtalidjE, or basket fish, the arms are branched (Astrophyton,* 

 fig. 319), but not, as usually stated, dichotomously. 



Fig. 318. Fig. 319. 



¥iG.'iK.—0]jhi<iphnViii aculeata* (From Morse.) 

 Fig. 'Al^.—Astniphytiin arburem-ens. basket fish. (From Ludwig.) 



Class III. Crinoidea (Pelmatozoa). 



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

 early times, especially in the paleozoic, they were verv abundant, 

 but to-day there are but few genera and species, these mostly 

 restricted to the greater depths of the ocean, only the Comatulida; 

 occurring near the shore. The crinoids are attached to the sea 

 bottom by a long stalk which contains a central canal (fig. 320). 

 This stalk is composed of cylindrical discs and often bears five rows 

 of outgrowths, the cirri. In the f'omatnlidre (fig. 321) the adult 

 is not thus attached, swimming about in the water with the arms 

 or moving about on the tang. In these earlier stages these animals 

 have a stalk (fig. 322), passing through a Pciitacriniis stage, a 

 jiroof that the fixed condition was the primitive one. In these 

 forms, when tlic separation takes place, one joint of the stalk with 

 its cirri remains attached to the animal, as the centrodorsal united 

 witli the lowest (mp plates, the infral)asals (fii,'-. 321). 



On the upper joint of the stalk is a cup-shaped body (theca) 

 the edges of which bear five or ten (usually branched) arms. The 



