130 
one of the least pleasures that an admirer of comparative anatomy enjoys, 
when he finds that results -which he acquired from one series of animals, wil} 
apply also to those which most nearly approach them in the analogy of their 
structure. The gradual increase of the plates or joints of the Crinoidea inclos- 
ing the abdominal cavity, took place by an extension at their lateral edges, to 
afford the room required by the viscera in the growth of the animal. The pro- 
ducing of auxiliary side arms, and reproducing of parts lost, demonstrate the 
formation of their joints, by a gradual secretion of calcareous matter from the 
involving muscular covering. These are facts which, since the Comatule are 
so nearly in conformity in their general structure to the Crinoidea, ought also to 
be demonstrable in their organization. 
The annular basin-shaped plate, which may be considered as the pelvis, 
appears principally to increase towards its upper margin, as it is on this side 
` only that new auxiliary side arms can be traced as growing out of it, hence its 
proportions become gradually deeper. "This pelvis is (as has been already 
observed) covered by numerous circular impressions, into which the side arms 
are inserted ; these circular impressions are surrounded by hexagonal rims, the 
result of an abundant secretion of calcareous matter from the muscular integu- 
ment which surrounds and strengthens the adhesion of these side arms at their 
base. The hexagonal form of these rims appears, as in many other instances, 
to arise from the mutual pressure and accommodation of the contiguous circles 
to each other. As this pelvis increases towards its upper edge, new arm joints 
are there protruded. I have in my possession specimens in which near the 
upper rim new cavities are formed, having in their centre minute, undeveloped, 
auxiliary side arms, entirely similar to those of Pentacrinus caput medusz, and 
where also the claw-like termination has not yet made its appearance. 
The limited number of Comatule which I have had an opportunity of 
examining, have not enabled me to ascertain whether the perforation in the ridge 
. which traverses each indentation in the pelvis passes through into theabdominal 
cavity, or whether the vessel arising from it branches off laterally, and obtains 
its supplies from vessels under the epidermis which covers the plates 
externally. 
Each of the five depressions at the upper rim ofthe pelvis is of a horse-shoe- 
like figure, in which the first costal joint rests, to this the second costal joint . 
adheres, and on that a scepulah is attached. The Two Costas much resemble 
