302 



ECHINODERMA 



radialia to unite in a complicated plexus in the centrodorsal. A problematical 

 so-called dorsal organ also begins in the centrodorsal and extends up through 

 the axis of the theca to the oral disc. It is apparently homologous with the 

 'heart' of other echinoderms. Its upper end lies in a cell complex from which 

 the reticulum of genital cords extends into the arms, swelling in the pinnuke 

 to the gonads (fig. 298). The dorsal organ in the centrodorsal is enclosed in 

 the chambered organ, a prolongation of the ccelom which extends into stalk ard 



cirri. 



Sub Class I. Eucrinoidea. 



The foregoing account applies entirely to the Eucrinoidea, which may be 

 divided into two groups: 



Order I. TESSELLATA (Pateocrinoidea). Theca with its side walls 

 composed of immovably united thin plates; the ambulacra! grooves usually 

 completely covered by calcareous plates. Exclusively paleozoic. 



a 



Fio. 207. FIG. 298. 



FIG. 297. Oral area of crinoid (Antedon), showing by dotted lines the course of the 

 intestine from the mouth (m) to the anus (a) ; g, ciliated grooves leading from the arms 

 to the mouth (prig.). 



FIG. 298. Cross-section of pinnula of Antedon (after Ludwig). a, axial nerve 

 cord; c, ciliated cups; cc, cceliac canal; g, gonad; s, sacculi; sc, subtentacular canal; t, 

 tentacles. 



Order II. ARTTCULATA (Neocrinoidea). Ambulacra! grooves open, 

 theca with compact, in part movably articulated, plates. This order left the 

 other in the mesozoic age, and some families have persisted until now. Rhizo- 

 crinus* (fig. 296) and Pentacrinus (fig. 294), deep seas; the COMATULID.*: of 

 shallow water are fixed in the young, free in the adult. Antedon* (fig. 295). 



Sub Class II. Edrioasteroidea (Agelacrinoided). 



Theca of irregular plates; arms unbranched and lying on the theca. Possibly 

 the ancestors of the noncrinoid echinoderms. Paleozoic. Agelacrinus. 



