76 THE VOYAGE OF H.M.S. CHALLENGER. 
interpalmar areas of the ventral perisome. They are continuous over the edge of the 
disk with the perisomatic plates uniting the rays; and he came to the conclusion that 
while many of the ventral plates are perforated by water-pores which lead downwards 
into the body-cavity, these openings are never found in the interradii at the sides of the 
disk. He termed them “anambulacral” to distinguish them from the “ ambulacralen 
Kelchporen fiir Fiisschen ;”? and this name has been conveniently extended both to the 
plates which they pierce, and also to the remaining imperforate plates of the interpalmar 
areas. Owing to the large size of the oral plates in Hyocrinus, which are themselves 
pierced by water-pores (Pl. Ve. fig. 6, wp), the number of these anambulacral plates on 
the disk is very small. But ina large Pentacrinus or Comatula they may be very 
extensively developed, and the pores are occasionally to be found on the sides of the disk 
between the rays (Pl. XVII. figs. 6, 10; Pl. XXVI. figs: djo2: IRL EXT Tig. 07% 
Pl. XXXIV. fig.i2; Pl XXXIX. fig. 2. Pl. LVII. figs. 1, 3, 4; Pl LIX. figs. 2,4, 6—wp ; 
EIS bexuD.): 
At the edge of the disk the anambulacral plates of its upper surface pass gradually 
downwards into the interradials, which are developed in the perisome uniting the rays; 
so that in some species both of Comatula and Pentacrinus the visceral mass is every- 
where protected by a continuous armour of plates. Many of the fossil Pentacrinide: and 
also some species of Apiocrinus show signs of the same structure. It is especially well- 
marked in the Liassic genus Extracrinus, which had a very large and thickly plated 
“ventral sac.” In fact the disk of these Crinoids seems to have borne stouter plates than 
that of many of the Paleozoic Ichthyocrinide ; and I do not understand how the ventral 
disk of this family, which is described by Wachsmuth? “as composed of a more or less 
soft or scaly integument, yielding to motion in the body and arms,” can be compared 
to anything else than the oral surface of a recent Crinoid, with which, however, 
Wachsmuth says that it “cannot in the remotest degree be homologised.*” 
I have not seen any good disk of Pentacrinus asteria; but, judging from the 
condition of its peripheral part in the specimen figured by Miller, I imagine it to have 
been covered with a continuous pavement of tolerably large plates. This is also the 
case in Pentacrinus wyville-thomsoni (Pl. XVIL. fig. 6). The interpalmar areas are 
covered with a very closely-fitting pavement of polygonal plates, the largest of which 
may be pierced by four or five water-pores, The anal tube, which is plated almost up to 
its summit, occupies the greater part of the corresponding interradius ; but the anambu- 
lacral plates which are between it and the mouth (in the specimen figured) are smaller 
than elsewhere, and less distinctly defined. In fact they look as if they had fused into 
two uwregularly-shaped plates which abut directly on the peristome. A similar fusion of 
small plates appears to have taken place on the anal tube of the Metacrinus nodosus 
1’ Bau des Pentacrinus, p. 49. 2 Bau der Echinodermen, p. 63. 
% Revision, pt.i. p. 31. 4 Amer. Journ. Sci., vol. xiv. p. 190. 
