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 " intcrvisceral " and the " circumvisceral " ccelom respectively. In some 

 Comatulse. such as Antedon rosacea and Actinometra strota,^ these two divisions of the 

 coelom are very distinctly separated ; while in other types, such as Antedon eschrichti, 

 Actinometra 2>cirvicirra, and also in tlie stalked Crinoids, it is difficult to fix a definite 

 boundary between them. In the former case the coiled digestive tube, wliich 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 Loven,* 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 julcesi. Several of these isolated disks were dredged 

 by the Challenger, together vvdtli a number of entire individuals and some eviscerated 

 calices ; 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 Criuoid vary con- 

 siderably. It may be more or less completely covered by plates (PI. YI. figs. 1-4 

 PI. XYII. figs. 6, 10; PI. XXYI. figs. 1, 2; PL XXX. fig. 2; PI. XXXIII. fig. 7 

 PI. XXXIY. fig. 2 ; PL XXXIX. fig. 2 ; PI. XLIIL fig. 3 ; PL L. fig. 2 ; PL LY. 



1 Beitriige zva Anatomie tier Crinoideen, ZcUschr. f. u-iss. ZooL, Bd. xxviii. pp. 306-308. 



2 The specific formula of Actinometra strota is — a . R . 10 . — . — 



' Proc. Roy. Soc. Land., vol. xxiv. pp. 213-215. 



* On Hyponome Sarsii, a recent Cystidean, Canadian Naturalist, N. S., vol. iv., 1869, pp. 265-268. 



* Proc. Eoy. Soc. Lond., vol. xxiv. p. 453. 



