DEATILOPMEXT OF AXTEDOX (C0:MATL'LA, LAMK.) ]{0SACEUS. 727 



59. Eeferring to the admirable Memoir of Professor Witillk Thomson' for full 

 details of the early development of the Pentacrinoid Larva from the free pseudembiyo, 

 I shall briefly recall the principal features which it presents ■Nvhcii it has fully assumed 

 the Crinoidal type. — ^The animal consists in tlie first instance of dcm and calyx alone, 

 not even rudiments of arms being distinguishable. The stem is composed of from 8 to 

 10 cylindrical segments, the lowest of which is articulated to a discoidal plate which 

 forms its base of attachment ; whilst the highest expands to serve as the foundation 

 whereon the plates of the calyx are built up. Tlie calyx completely encloses the visceral 

 mass, its oral valves, when drawn together, meeting over the mouth ; whilst, when these 

 open out, it has the form of an inverted belP. Its lower or aboral portion is composed 

 of five hasal plates, Avhich form by their approximation a five-sided pyramid witli its 

 truncated apex pointing downwards ; whilst its oral part is composed of five oral 

 plates, the approximation of which forms a similar pyramid with its truncated apex 

 pointing upwards, though they are usually erected as separate valves, to allow the oral 

 apparatus to be projected from within them. These two pyramids are the parts of the 

 skeleton of the calyx which first make theii' appearance ; and the one is so supeqiosed 

 on the other that the plates of the oral are oj)posife to those of the basal pyramid. 

 At a somewhat later period the jftrst raduds make their appearance in the spaces left by 

 the contiguous angles of the basal and oral plates, so as to alternate with these in posi- 

 tion. Between two of the radials, and on the same level with them, an unsymmctrical 

 plate early shows itself, the subsequent relation of which to the vent proves it to be an 

 anal plate. From the summits of the first radials, the second radials are next seen to 

 bud forth between the bases of the orals ; and as the circle formed by these last does 

 not increase in diameter, whilst the ring of first radials on which it rests has become 

 larger, the orals are relatively carried inwards, whilst the second radials project somewhat 

 outwards. — In the latest stage described by Professor Wyville Thomson, the third or 

 axiUary radials have begun to show themselves at the distal extremities of the second ; 

 and rudiments of a pair oi first brachials are distinguishable at the distal extremities 

 of the third radials^ 



60. The several portions of the skeleton are imbedded in a nearly homogeneous sar- 

 codic substance, which externally forms a perisomatic investment, and internally consti- 

 tutes a lining to the calyx. The Digestive Cavity seems in the first instance to be simply 

 excavated in the body-substance ; but as the radial plates are developed, giving an 

 expansion to the equatorial portion of the cup, the wall of the Stomach becomes sepa- 

 rated from the lining of the calyx by a distinct perivisceral cavity filled with fluid, m 

 which the stomach seems to hang attached to the body-wall here and there by sarcodic 

 bands and threads. Simultaneously with the appearance of the anal plate, a slender digi- 

 tate process rises from one side of the stomach and curves towards that plate ; this con- 

 stitutes the rudiment of the Intestine, but it has not as yet any outlet, and I believe it 

 to be in this stage destitute even of an internal canal, being an extension of the «'«// of 

 ' Philosophical Transactions, 1865, p. oV-i. ■ Ihid. Plate XXVI., figs. 1, 2. - Ibid. Plate XXYII. fig. 1. 



JlDCCCLXVr. 5 G 



