1870.] BILLINGS— ON CRINOIDEA AND BLASTOIDEA. 181 



Professor Wyville Thompson has proposed a division of the 

 skeleton of the existii\2; Crinoid, Antcdon rosaceus into two sys- 

 tems of plates, which he terms respectively the " i?«cZia/," and 

 the " Fcrisomatlc " systems.^ These he considers to be thor- 

 oughly distinct from each other in their structure and mode of 

 growth. The radial system consists of the joints of the stem, 

 the centrodorsal plate, the radial plates, the joints of the arms, 

 and also those of the pinnules. In the perisomatic system he, 

 includes the basal and oral plates, the anal plate, the interradial 

 plates, and any other plates or spicula which may be developed 

 in the pcrisome of the cup or disc. This I think a good arrange- 

 ment, except in so far as it regards the stem, which appears to 

 me to be, always, an appendage of the perisomatic, rather than 

 of the radial system. 



Throughout the whole range of the Criuoidea, the plates of the 

 radial and perisomatic system, 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 ambu- 

 lacral orifice. This accords well with an important observation 

 of Professor Thompson's on the structure of Antedon, while in 

 the earlier periods of its growth. " The entire body of the Pen- 

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

 pscudembryo and during its earliest fixed stage, surrounded 

 and inclosed by plates of the perisomatic system alone, and it is 

 quite conceivable that plates belonging to this system may ex- 

 pand and multiply so as to form a tessellated external skeleton 

 to the mature animal, the radial system being entirely absent, 

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

 5-11). Such is the structure of all of the Cystidea. On refer- 

 ring to fig. 2, it will be seen that the whole of the body of Carijo- 

 cijstites tcstudiiiarius, is coverered 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 sub- 

 cylindrical Cystideans), annulated rather than radiated, so far as 

 regards the external integument. The lower range, below the 

 line, h, consists of the basals, whilst the upper, above the line, r, 

 may possibly, be radiated. In all the globular or ovate Cysti- 



*0n the Embryogeny of Antedou rosaceus Linck (Comatula rosacea 

 of Lamarck). By Professor "Wyville Thompson, L.L.D., &c. Philo- 

 sophical Transactions of the Eoyal Society, vol. civ, Part II, p. 540. 



