100 THE VOYAGE OF H.M.S. CHALLENGER. 
Towards the hinder edge of the mouth the trabecular structures in the left division of 
the labial plexus become more and more delicate, their cellular covering thins out, and 
the true spongy organ appears, with the characters described above. It is formed almost 
entirely from the network on the left side of the mouth in which the plexiform gland ends 
(Pl. LIX. fig. 9, vv). It remains throughout nearer to the left than to the right of the 
two posterior ambulacra, gradually becoming more compact again, and finally passes into 
the plexus of intervisceral and genital vessels. 
The spongy organ of the type which is generally called Antedon rosacea varies very 
much in its structure. In some cases it is almost as reticular as in Antedon eschrichti, 
while in others it is hardly differentiated from the rest of the labial plexus, and consists 
of a mass of twisted tubules, which have well defined epithelial walls. It is possible 
that these variations may be due to specific differences, but upon this point I can, as yet, 
offer no opinion. 
The labial plexus of Pentacrinus, at any rate of the two species which I have studied 
(Pentacrinus decorus and Pentacrinus wyville-thomsoni), is much more highly developed 
than in the Comatule, but it contains nothing like the spongy organ of Antedon eschrichti 
and its allies. It extends outwards for some little distance from the peristome, both 
beneath and between the ambulacra. In the former case it is connected with the radial 
blood-vessels, beneath the middle line of the groove, thus keeping apart the converging 
water-vascular trunks at its sides as already described (Pl. LVIL. figs. 1, 3, 4, 77). 
Its histological condition in the individuals of both the species which I have examined 
is not such as to facilitate the observation of minute structural details, but from what I 
have seen of the better preserved portions of it, I have no reason to think that it differs 
essentially from the corresponding organ in Antedon. The epithelial lining of its cavities 
is often fairly distinct, as shown in Pl. LVII. fig. 4. 
As the ambulacra recede from the peristome and thus diverge more and more, the 
vascular plexus underlying the interpalmar area which separates them gradually thins 
out, until it is only represented by the uppermost intervisceral vessels and the genital 
vessels of the rays, both of which originate init. The former belong to the cireumvisceral 
layer of the peritoneum, and the latter to the parietal layer ; but the two systems are in 
free communication with one another (Pl. LVII. fig. 3, gv, ib). 
The tubules depending from the radial blood-vessel and entering the subambulacral 
plexus gradually become less and less abundant, and finally disappear altogether, shortly 
before the limit of the water-tubes is reached. In the anal interradius the labial plexus 
is specially developed, as is well shown in transverse section in Pentacrinus wyville- 
thomsoni (Pl. LVII. fig. 1, 7p), lateral extensions of it being connected with the radial 
blood-vessels by the tubules above mentioned. [See Appendix, Note D.] 
The subdivision of the upper end of the plexiform gland into numerous branches 
which terminate in the labial plexus, can generally be made out without any difficulty 
