32 DR. p. H. CAEPENTEE ON CERTAIN POINTS 



i. e. the supposed kidney. On similar grounds, too, we might 

 regard the lateral pyramid of Agelacrinus as the common oscular 

 orifice of the nephridial, genital, and digestive systems. 



The above argument is based on the supposition that the Cystids 

 had an ovoid gland (kidney, Sarasin) like the Crinoids and Urchins ; 

 but there is also the possibility that in some among them, e. g. 

 the less Crinoid-like forms, such, as Caryocystis and Megacystis, 

 the excretory and amoebiform functions of the ovoid gland were 

 performed by the so-called " water-lungs," as seems to be the 

 case in the Holothuriaus with no external madreporite. These 

 organs open into the cloaca, together with the rectum, of which 

 they are primitively diverticula, and the cloacal opening (anus) is 

 more or less protected by valvular plates which represent the 

 pyramid of the Cystids. In either case, therefore, it seems probable 

 that the lateral pyramid o? Agelacrinus, CyatJiocystis, Caryocrinus, 

 and similar forms may have been both excretory and anal in func- 

 tion ; while the analogy of Hymenaster and Pytlionaster would 

 suggest that it also served as the outlet of the genital products, 

 so that these types with only one recognizable opening besides 

 the mouth might be fairly described as Cystidean Monotremes. 



3. Some G-eneral Considerations. 



I have endeavoured to show in the early part of this paper 

 that the dorsal cup of many Cystids is composed of plates which 

 correspond respectively to the infrabasals, basals, and radials of 

 a Criuoid. In former memoirs * I have likewise pointed out that 

 these plates may be recognized in the larvse of Asterids and 

 Ophiurids, and also in many adults of both classes f. Dorsocentral 

 basals, and perhaps radials, occur in the larval Echinid, and all 

 persist in the adults of some generic types ; though in others 

 only the basals and radials are traceable, as in the Blastoids, 

 which we may fairly assume to have had a dorsocentral at the 

 base of the stem, just like the young Crinoid, and the same may 

 be said of the stalked Cystideans. It is curious, however, that 

 infrabasals, which are so frequently developed in the brachiate 

 forms, should be unknown in the Urchins and also in the Blas- 

 toids, neither class possessing definite appendages in which the 

 ambulacra terminate ; and their absence in the Blastoids is the 



* See more especially the chapter " On the Homologies of the Orinoidal Calyx 

 in the other Echinoderms," Report on the Crinoidea, Zool. Ohall. Exp. vol. xi. 

 1885, pp. 393-402. 



t See postscript, infva, p. 44. 



