REPORT ON THE CEINOIDEA. 99 



spongy organ is a somewhat egg-sliaped mass, consisting almost entirely of a delicate net- 

 work of connective tissue with more or less open meshes. 



The latter are rather wider in Promachocrimis than in any other Crinoid which I 

 have examined. The trabeculse 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 

 trabeculse and generally smaller meshes. These are lined by epithelial cells, beino' 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 (PI. LX. fig. 5). 



Some of these vessels are seen in more or less oblique section at the rio-ht end of the 

 spongy organ of Fromachocrinus kerguelensis (PI. 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 Promachocrimis. 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- 

 ^dsceral 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 trabeculiB 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 tyj^e 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 cud of the plexiform gland, which retains its 

 individuality till quite close to the mouth (PI. LIX. fig. 9, xr), for it remains large and 

 lobulated, instead of breaking up as in Antedon ; and the connection of its reticidar 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. 



