f^() THE VOYAGE OF H.M.S. CHALLENGER. 



formed, like the anambulacral plates, of a calcareous network interpenetrated l)y an 

 organic basis, Avliich is of the same nature as in the joints of the rays and arms (Pis. 

 LVIL, LXIL). 



The simple spicules and thin networks of limestone which occur in the less heavily 

 plated disks are especially abundant in the visceral layer of the peritoneum. This is well 

 seen in those Crinoids, such as Antedon rosacea^ and Actinometra strata, in which there is 

 but little connection between this visceral layer and the parietal one lining the interior of 

 the cup, so that the entire visceral mass is readily detached from the calyx. 



Unlike that which lines the cup, the oral perisome is usually very closely adherent 

 to the visceral mass, and cannot be separated from it without some trouble. The 

 peritoneal covering of the latter also contains limestone deposits, so that sections 

 through the upper part of the disk show two layers of plates and spicules. The 

 upper one is in the perisome itself, and belongs to the anamlDulacral system ; while 

 the lower belongs to the upper surface of the visceral mass. These lower plates were 

 described and figured liy Mliller.^ Together with those of the sides and lower 

 parts of the visceral mass they seem to be the modern representatives of the 

 so-called " intervisceral plexus " which lined the cup of the Actinocrinidse. Wachsmuth ^ 

 has pointed out that in some members of this family " almost thi' entire test is lined 

 with a delicate calcareous plexus or network. This lining is not in contact with the test 

 directly, Init connected with it by small partitions, producing innumerable little chambers, 

 which communicate with each other and with the visceral cavity." It rarely extends 

 lielow the level of the second radials, and passes gradually upwards into the plates, 

 coating the interpalmar areas on the ujjper surface of the disk below the vault. These, 

 which extend right up to the edge of the peristome, are of course anambulacral in 

 character, and it is not easy, any more than it is in Pentacrimis, to say where the one 

 set begins and the other ends. But so fcir as the lower part of the cup is concerned, 

 the intervisceral plexus of the Actinocrinidse is merely a greater development of the 

 limestone deposits in the visceral layer of the peritoneum of recent Crinoids. 



All of these have more or less abundant plates and spicules in the connectiA-e tissue 

 which lies beneath the peristome and supports the lip, and also in that which unites 

 the coils of the digestive tube (PL LXIL). In the regular endocyclic Crinoids this 

 organ makes rather more than a single round turn upon itself (fig. 2 on p. 89) ; and it 

 is the aggxegation of limestone deposits upon its central side which forms the so-called 

 " columella," once regarded as a sand canal. 



This supporting skeleton of the digestive tulse, lUce that enclosing the visceral mass, 

 was much better developed in the Actinocrinid?e than in recent forms. Occupying 



' Liulwif,' (ojj. cit., p. 3311, Taf. xix.) has criven an excellent diagrammatic section of this type, in whicli this 

 point is well illustrated. 



- Ban der Echinodermen, p. 58, Tat. vi. fi;;. 9,/. ^ Revision, part ii. p. 2(). 



