Ill 



Fig. 71. Fig. 72. 



Fig. 71.— Calj'cine plates of Pentremites, — b, the basals ; /, one of the five forked plates ; 

 d, deltoid plate ; /, lancet plate ; os, oral spiracle ; s, spiracle. 

 Fig. 72. — Caryocystites testudinarius, Hisinger, — b, basal plates ; r, radials ; m, mouth. 



basal and oral plates, the anal plate, the interradial plates, and any other 

 plates or spicula which may be developed in the perisom of the cup or 

 disc. This I think a good arrangement, except in so far as it regards 

 the stem, which appears to me to be, always, an appendage of the peri- 

 somatic, rather than of the radial system. 



Throughout the whole range of the Crinoidea, the plates of the radial and 

 perisomatic systems, are easily distinguished from each other. In general, 

 the Cystidea have no radial plates in their calyces except, perhaps, in a 

 small area around the ambulacral orifice. This accords well with an im- 

 portant observation of Professor Thomson's on the structure of Antedon, 

 irV'hile in the earlier periods of its growth. The entire body of the Pen- 

 tacrinoid is," he says, " at first, while yet included within the pseudem- 

 bryo and during its earliest fixed stage, surrounded and enclosed by plates 

 of the perisomatic system alone, and it is quite conceivable that plates 

 belonging to this system may expand and multiply so as to form a tessel- 

 lated external skeleton to the mature animal, the radial system being en- 

 tirely absent, or represented only in the most rudimentary form." (Op. 

 cit., p. 541). Such is the structure of all of the Cystidea. On referring 

 to fig. 72, it will be seen that the whole of the body of Canjocijstitcs testu- 

 (Unarius, is covered with polygonal plates, without any trace whatever of 

 a radiated arrangement. The plates are disposed in nine transverse 

 ranges, girding the body like so many rings. This species is, (and so 

 are most of the elongated subcylindrical Cystideans), annulated rather 

 than radiated, so far as regards the external integument. The lower 

 range, below the line, 6, consists of the basals, whilst the upper, above 

 Ihe line, r, may, possibly, be radiated. In all the globular or ovate Cys- 



