REPORT ON THE CRINOIDEA. 73 



B. The Perisomatic Skeleton. 



This name was given by Sir Wyville Thomson^ to " the basal and oral plates, the 

 anal plate, the iuterradial plates, and any other plates or spieula which may be developed 

 iu the perisome of the cup or disk." He pointed out that the plates of this system are 

 " essentially variable in number and arrangement ; most of the minor structural modi- 

 fications throughout the group depend upon the multiplication or suppression of plates 

 of this series. Even in the same species they are by no means constant," e.g., Antedon 

 rosacea. The nature of the basals and orals has been already discussed; and very little 

 need be said about the anal plate. For although this forms an essential part of the cup 

 of the Pentacrinoid larva of Comatula, and is of extreme importance in its palasontological 

 relations, yet it disappears soon after the termination of Pentacrinoid life, undero^oino- 

 exactly the same process of resorption as the orals have previously done. It is curious, 

 however, that there should be no special anal plate in Hijocrinus, which has such large 

 orals (PL VI. figs. 1-5), while it is also absent in the adult Rhizocrinus, and is perhaps 

 never developed at all ; for Sars figures a young individual only 25 mm. long in which 

 the first brachials are comparatively large and form a sort of pyi-amid, while the second 

 brachials are undeveloped, and he makes no mention whatever of an anal plate. ^ Whereas 

 in Antedon rosacea the anal plate appears soon after the second radials (which represent 

 the first brachials of Rhizocrinus) ; and it is relatively quite large by the time that the first 

 brachials are developed, forming a nearly complete circle together with the first radials, 

 between two of which it is intercalated. 



The interradial plates are those minute disks or granules which occur in the substance 

 of the perisome uniting the rays and their subdivisions, and are sometimes difficult to 

 distinguish from the lowest joints of the pinnules. They were first detected in Antedon 

 milleri by J. S. Miller,^ who figured them as forming one " intercostal " between every 

 two second radials. This was probably due, as remarked by Dr. Carpenter,* to his having 

 only employed a low magnifying power in his examination of them. Miiller ' described 

 them as occurring in Pentacriniis asteria (PL XIII. fig. 1), and noticed their difi'erence from 

 the plates on the ventral surface of the disk which are pierced by the water-pores 

 (PL XVII. figs. 6, 10). They are very abundant in some species of the Comatulidte and 

 Pentacrinidse, uniting the rays and their lowest divisions very closely together ; while in 

 other t}'pes they may be whoUy or entii'ely absent in some individuals, and more or less 

 well developed in others. In fact, the same individual may have them in one or two of 



1 Phil Tram., 1865, pp. 540, 541. 



2 Crinoides vivants, p. 27. Tab. iv. fig. 95. 



3 This is the Comatula fimhriata of Miller, which occurs in Milford Haven. See his Natural History of the Crinoidea, 

 Bristol, 1821, Frontispiece, fig. 2, G. 



* Phil. Trans., 1866, p. 716. ' Bau des Pentacrinus, loc. cit., p. 49. 



(ZOOL. CHALL. EXP. — PAET XXXII. — 1884.) 11 10 



