l90 p. HERBEET CARPENTER. 



water-pores being very numerous, and in Caryocrinus^ the 

 pores are also numerous, but " nehmen den antiambulacralen 

 Theil des Kelcbes hinter den Armen bis zur Basis ein." 

 On the other hand, Ludwig^ has shown that in RJiizocrmus 

 there is but one water-pore in each interradial area of the 

 disc, although the plating of the perisome may be very exten- 

 sive. Water-pores are found in most Cystids, being variously 

 arranged into poriferous or tubular structures, but the distri- 

 bution of these is very different in different genera. They are 

 usually antiambulacral, as in Caryocrimis ^ but in Protocrinus 

 and Glyptosplicerites they occur between the ambulacra of 

 the ventral surface as in Pentacrinus. Billings^ has at- 

 tempted to shoAv " the gradual passage or conversion of the 

 respiratory organs of the Cystidea, Blastoidea, and Palao- 

 crinoidea into the ambulacral canal system of the recent 

 Echinoderms." I cannot, however, regard his attempt as 

 successful. This is not the place for a critical examination 

 of his arguments, but I may remark that the pectinated 

 rhombs of the Cystids and the hydrospires of the Blastoids 

 are all interradial, while the water-vasoular trunks of the 

 recent Echinoderms are radial in their origin. Further, 

 Miiller has pointed out that the homologues of the former in 

 the recent Crinoids are the water-pores of the disc, the exist- 

 ence of which appears to have been quite unknown to 

 Billings; and Ludwig^ has recently shown the very close 

 resemblance between the structure of the so-called genital 

 clefts of the Ophiurids and the hydrospires of Pentatremites, 

 so that there is no occasion to seek for the homologues of the 

 latter in the water-vascular system of the Crinoids or 

 Ophiurids as Billings has done. 



The skeleton of a modern Crinoid then, may be regarded 

 as composed of three distinct systems of plates, viz. the 

 abactinal or apical, the actinal or oral, and the intermediate, 

 which is both ambulacral and inter- or anambulacral. These 

 may be developed in very different degrees of complexity, 

 especially in the older forms. Their mutual relations are 

 presented as simply as possible in the modern Hyocrmus, 

 while in genera like Euccdyptocrinus and RJwdocrinus, one 

 or more of these systems is extremely complicated by the 

 extensive subdivision of its primary elements and the de- 

 velopment of secondary ones. Lastly, in Comahda the oral 



^ ' Bau der Echiuodermen,' p. 64. 



2 Loc. cit., p. 118. 



3 ' Cauad. Nat.,' 1869, p. 426. 



■» "BeitragezurAuatomie der Opbiuren," ' Zeilsclir, fur wiss. Zool.,' Bd. 

 sxxi, pp. 2S2-2S5. 



