78 THE VOYAGE OF H.M.S. CHALLENGER. 
contained in it are continued from one joint to another between the two large muscular 
bundles that unite them. In Antedon eschrichti and in many other Comatulz, more 
especially those belonging to the genus Actinometra, this arm-groove merely lodges the 
lowest part of the cceliac canal; while the genital cord, with the water-vascular and 
blood-vascular trunks and the ambulacral epithelium, are all situated above the arm- 
groove, and separated from it by a variable amount of intervening perisome, so that little 
more than half the vertical height of the arm is due to its dorsal skeleton. The lower 
parts of the arms in Metacrinus murray: present a somewhat similar condition 
(Pl. XLI. fig. 13). 
In other Comatulz, however, and in Pentacrinus a great part, sometimes even the 
whole, of the soft parts of the arm are lodged within the groove on the upper surface 
of the skeleton (Pl. XVII. figs. 1, 4; Pl. XXVII. fig. 6); and there is no substantial 
ventral perisome in the ordinary sense of the word, or it is reduced to a mere film, 
sometimes thinly plated, which covers up the muscular bundles. In many species, and 
especially in the small deep-sea Comatulee, this layer of perisome is excessively thin and 
transparent, so that the food-groove appears to rest upon and between the muscular 
bundles. In some of the tropical Antedons, however, it bears a continuation of the 
anambulacral plates of the disk, and this is also the case in Pentacrinus wyville-thomsoni, 
Pentacrinus alternicirrus, Pentacrinus naresianus, and Pentacrinus blaket (Pl. XVII. 
fig. 4; Pl. XXVII. figs. 6,13; Pl. XXXIIL. fig. 3). The third of these, Pentacrinus 
naresianus, has the greatest development of this plated perisome on the arms (Pl. XXVII. 
fig. 13). It is continuous from one pinnule socket to the next on the same side, so as 
to cover in both the muscular bundles and also the upper surface of the intervening 
arm-joint; and the ambulacra are thus distinctly above and outside the arm-groove. 
They are bordered by large oval covering plates which overlap alternately from opposite 
sides, and are continued on to the pinnules (Pl. XXVII. figs. 11, 12). These plates do 
not rest directly upon the pinnule-joints, but are separated from them by a thin 
limestone band which is a continuation of the lateral plating of the arm. It does not, 
however, exhibit any differentiation into side plates, though its edges are cut out into 
alternate teeth and notches (Pl. XXVII. fig. 11). The latter are occupied by the 
— tentacles, but can be closed, or nearly so, by the covering plates which rest on the 
intervening teeth. 
In the arms of Pentacrinus blakei (Pl. XXXIII. fig. 3) the sides of the joints bend 
inwards towards the middle line more than they do in Pentacrinus naresianus, so that 
the arm-groove is narrower, and the ambulacrum practically coincides with it instead 
of lying above it. It is bordered by long plates which are really the covering plates 
fused with the side plates. When they pass on to the pinnules the former become 
more differentiated, but the latter lose their individuality and become parts of a con- 
tinuous denticulated band just as in Pentacrinus naresianus (Pl. XXXIIL. fig. 1). 
