REPORT ON THE CRINOIDEA. 99 
spongy organ is a somewhat ege-shaped mass, consisting almost entirely of a delicate net- 
work of connective tissue with more or less open meshes. 
The latter are rather wider in Promachocrinus than in any other Crinoid which I 
have examined. The trabeculee forming its outer portion are much more delicate than 
those nearer the surface; and I have been unable to make out that they possess 
any epithelial covering. The surface of the organ, however, is more compact, with larger 
trabeculae and generally smaller meshes. These are lined by epithelial cells, being in 
fact the ends of those blood-vessels forming the labial plexus which are connected with 
the spongy organ, mostly, if not entirely, on its ventral side (Pl. LX. fig. 5). 
Some of these vessels are seen in more or less oblique section at the right end of the 
spongy organ of Promachocrinus kerguelensis (Pl. LIX. fig. 8), while the relation of the 
spongy organ to the labial plexus generally is well shown in the transverse and longi- 
tudinal sections through the disk of Antedon eschrichti (Pl. LX. figs. 3, 5). The spongy 
organ of this species is more compact than that of Promachocrinus. It is similarly 
situated in the space left by the incomplete adhesion of the visceral and parietal layers 
of the peritoneum ; and it is suspended in this space by threads of connective tissue. It 
is practically the direct backward continuation of the labial plexus at the eastern angle 
of the mouth, where it is much more largely developed than on the opposite side. The 
relatively thick epithelial wall of the vessels gradually disappears as they enter the 
spongy organ ; while the latter in its turn passes insensibly backwards into the plexus of 
vessels on the upper surface of the visceral mass, from which are derived both the inter- 
visceral vessels and the genital vessels of the two posterior ambulacra. The absence of 
an epithelial lining in the spaces of the spongy organ is very marked, although the 
epithelium is quite distinct in the blood-vessels which terminate therein. But, on the other 
hand, I have found, both in this species and in Antedon quadrata, that the nuclei of the 
connective tissue forming the trabeculee stain very prominently, much more so than I 
could get them to do in Promachocrinus kerguelensis. 
Although the simple reticular structure of the spongy organ in this latter type is 
limited, as described above, to that part of the labial plexus which is situated between 
the mouth and anus, yet the distinction between it and the remainder of the labial plexus 
is far less sharp than in Antedon eschrichti. For the vessels forming the labial plexus of 
this type are much more closely grouped, and have a less definite epithelial wall than is 
the case in Antedon eschrichti, so that it assumes a decidedly reticular character. 
This is also the case with the upper end of the plexiform gland, which retains its 
individuality till quite close to the mouth (Pl. LIX. fig. 9, wv), for it remains large and 
lobulated, instead of breaking up as in Antedon ; and the connection of its reticular portion 
with the labial plexus at the north-east angle of the mouth is quite distinct ; while at the 
western angle the labial plexus, though somewhat reticular in structure, is but poorly 
developed. 
