728" DJL W. B. CAEPENTEE ON THE STRTJCTUBE, PHYSIOLOGY, AND 



the stomach but not of its cavity. — ^The mouth of the bell, when the oral valves are 

 expanded, is entirely occupied by the oral apparatus. The mouth itself is a mde orifice, 

 allowing the interior of the stomach to be plainly seen when looked into from above ; 

 and this is surrounded by a prominent ring from which arise the oral tentacles'. This 

 ring is spoken of by Professor Wyville Tiiomsox as a proper " vessel ; " but, as I 

 shall show hereafter, although it doubtless represents the vascular ring which sun-ounds 

 the mouth of Echinoderms generally, it is really an extension of the perivisceral cavity, 

 partly separated from the rest. The oral tentacles are of two kinds, extensile and 

 non-extensile. Of the former there are in the first instance only five, presenting 

 themselves at the intervals between the oral valves; whilst of tlie latter there are ten, 

 a pair lying within each of those valves. Coincidently with the development of the 

 Radials, however, the first-formed extensile tentacles are carried outwards on diver- 

 ticula from the oral ring, which thus originate the tentacular canals of the arms ; wliilst 

 five additional pairs of extensile tentacles are developed between the five pairs of non- 

 extensile, raising the whole number to twenty-five, of which the five first-formed are a 

 little external to the rest. With the development of the second and thud Radial plates 

 that of the canal-system of the rays proceeds. The five " azygous " extensile tentacles 

 are gradually carried outwards, by the prolongation of their respective diverticula, to the 

 point of bifurcation of the Kay into Arms ; and from these diverticula, which now con- 

 stitute the tentacular canals of the rays, there arises on either side a series of delicate 

 crescentic leaves, each of them having in connexion Avith it one extensile and two non- 

 extensile tentacles'' ; an arrangement which afterwards comes to be repeated along the 

 arms when they are developed at the bifurcation of the rays, and subsequently along the 

 pinnules which fringe the arms of the adult. Beneath the tentacular canal a tubular 

 extension of the perivisceral space passes along the ventral sui'face of each ray ; and 

 although this appears to form but a single canal, I shall hereafter show that it is 

 very early divided by a horizontal partition extended from the membranous bands 

 that suspend the digestive cavity in the perivisceral space ; and that whilst the canal 

 above the partition communicates with the portion of the peri-visceral space which lies 

 immediately round the mouth, the canal beneath the partition is extended from the 

 portion of the perivisceral space which occupies the hollow of the calyx. The former I 

 shall term the subtentacular, and the latter the coeliac canal; their relations will be 

 found very remarkable.— The general aspect of the young Pentacrinoid, as seen with its 

 tentacular apparatus retracted in a spirit-specimen, is shown in Plate XXXIX. fig. 1, a, 

 for the sake of comparison with the later stages. 



61. For some little time after the appearance of the arms, the relations of the skeleton 

 of the calyx to the visceral mass it includes undergo but little change (Plate XXXIX. 

 fig. 1, b); the chief difference consisting in the more compact condition it now comes 

 to present, in consequence of the advanced development of its component pieces. The 

 five basals (Plate XLI. fig. 1, b, b) now possess a regularly trapezoidaL form ; the 

 • > PhUoaophical Transactions, 1865, Plate XXVI. fig. 3. ^ lUd. Plate XXYH. fig. 3. 



