REPORT ON THE CRINOIDEA. 67 
V.—THE VISCERAL MASS. 
The body-cavity enclosed between the dorsal skeleton and the ventral perisome of a 
Crinoid consists of two principal parts, which have been conveniently designated by 
Ludwig! as the “ intervisceral ” and the “ circumvisceral” ccelom respectively. In some 
Comatulee, such as Antedon rosacea and Actinometra strota, these two divisions of the 
ccelom are very distinctly separated ; while in other types, such as Antedon eschrichti, 
Actinometra parvicirra, and also in the stalked Crinoids, it is difficult to fix a definite 
boundary between them. In the former case the coiled digestive tube, which is covered 
in above by the ventral perisome, is protected below by a continuous sheet of connective 
tissue. This forms a definite membrane enclosing the lower part of the visceral mass or 
disk, and has been spoken of as the visceral layer of the peritoneum.’ In Antedon rosacea, 
Actinometra strota, and similar forms, this visceral layer is only loosely attached to the 
parietal layer which lines the interior of the cup formed by the rays and arm-bases. The 
result is that a comparatively slight amount of violence is sufficient to separate the 
visceral mass from the calyx and to tear the ambulacra across at the arm-bases. The 
whole visceral mass, including the digestive tube and plexiform gland, together with the 
circumoral rings of the blood-vascular, water-vascular, and ambulacral nervous systems, 
is very apt to be turned out of the calyx, which is then left to swim about on its own 
account. 
The so-called “recent Cystidean,” Hyponome sarsii of Lovén,* is, in fact, nothing 
more than the much-plated visceral mass of an Antedon common at Cape York (PI. LV. 
figs. 3, 4); and the same thing may happen to the disks of the equally abundant 
Actinometra strota and Actinometra jukesi. Several of these isolated disks were dredged 
by the Challenger, together with a number of entire individuals and some eviscerated 
ealices ; and Sir Wyville Thomson informed me that he had observed them perform slow 
movements of locomotion over a flat surface; while we know from Dr. Carpenter's 
experiments that the eviscerated but arm-bearing calyx of Antedon rosacea will execute 
the usual graceful movements of swimming as perfectly as the entire animal had 
previously done.’ 
The characters of the perisome covering the ventral surface of a Crinoid vary con- 
siderably. It may be more or less completely covered by plates (Pl. VI. figs. 1-4; 
Pieeevliencs. 6,000; PU XX Vi tes, 1, 2 Pl XXX fie, 2); Pl’ XXXII. fig? 7; 
Pipe ia, 27 EN eR NIX fio 25 PIXEL fig. 3 PLL. fig. 25 PL LVo; 
1 Beitrage zur Anatomie der Crinoideen, Zeitschr. f. wiss. Zool., Bd. xxviii. pp. 306-308. 
2 The specific formula of Actinometra strota is—a.R.10. at — : 
2 
5 Proc. Roy. Soc. Lond., vol. xxiv. pp. 213-215. 
+ On Hyponome Sarsii, a recent Cystidean, Canadian Naturalist, N. S., vol. iv., 1869, pp. 265-268. 
5 Proc, Roy. Soc. Lond., vol. xxiv. p, 453, 
