86 THE VOYAGE OF H.M.S. CHALLENGER. 
formed, like the anambulacral plates, of a calcareous network interpenetrated by an 
organic basis, which is of the same nature as in the joints of the rays and arms (Pls. 
LVIIL, LXIZ.). 
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 strota, 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 anambulacral system; while 
the lower belongs to the upper surface of the visceral mass. These lower plates were 
described and figured by Miiller.? 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 Actinocrinide. Wachsmuth * 
has pointed out that in some members of this family “almost the entire test is lined 
with a delicate calcareous plexus or network. This lining is not in contact with the test 
directly, but connected with it by small partitions, producing innumerable little chambers, 
which communicate with each other and with the visceral cavity.” It rarely extends 
below the level of the second radials, and passes gradually upwards into the plates, 
coating the interpalmar areas on the upper 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 Pentacrinus, to say where the one 
set begins and the other ends. But so far as the lower part of the cup is concerned, 
the intervisceral plexus of the Actinocrinide 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 connective tissue 
which lies beneath the peristome and supports the lip, and also in that which unites 
the coils of the digestive tube (Pl. LXII.). 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 aggregation 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 tube, like that enclosing the visceral mass, 
was much better developed in the Actinocrinide than in recent forms. Occupying 
1 Ludwig (op. cit., p. 330, Taf. xix.) has given an excellent diagrammatic section of this type, in which this 
point is well illustrated. 
2 Bau der Echinodermen, p. 58, Taf. vi. fig. 9, f- 3 Revision, part ii. p. 26. 
