78 THE VOYAGE OF H.M.S. CHALLENGEK. 



contained in it are continued from one joint to another between the two large muscular 

 bundles that unite them. In Antedon eschriehti and in many other Comatulte, more 

 especially those belonging to the genus Actinometra, this arm-groove merely lodges the 

 lowest part of the coeliac 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 murrayi present a somewhat similar condition 

 (PL XLI. fig. 13). 



In other Comatulse, 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 (PI. XVII. figs. 1,4; PI. 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 Comatulse, 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 unjville-tliomsoni, 

 Pentacrinus cdternicirrus, Pentacrinus naresianus, and Pentacrinus hlakei (PL XVIL 

 fig. 4; PL XXVIL figs. 6, 13; PL XXXIII. fig. 3). The third of these, Pentacrinus 

 naresianus, has the greatest development of this plated perisome on the arms (PL XXVIL 

 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 XXVIL 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 XXVIL 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 hlakei (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 -n-ith the side plates. When they pass on to the j^innules the former become 

 more difi"erentiated, but the latter lose their individuality and become parts of a con- 

 tinuous denticulated band just as in Pentacrinihs naresianus (PL XXXIII. fig. 1). 



