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 

 (PI. LIX. fig. 9, xi'). 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, 

 whUe 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 difi'erences, but upon this point I can, as yet, 

 ofi"er no opinion. 



The labial plexus of Pentacrinus, at any rate of the two species which I have studied 

 (^Pentacriniis decorus and Pentacrinus wyville-thomsoni), is much more highly developed 

 than in the ComatulEe, 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 (PI. LVII. figs. 1, 3, 4, Ir). 



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 difiers 

 essentially from the corresponding organ in Antedon. The epithelial lining of its cavities 

 is often fairly distinct, as sho'WTi in PI. 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, untU it is only represented by the uppermost inter\'isceral vessels and the genital 

 vessels of the rays, both of which originate in it. The former belong to the circumvisceral 

 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, ih). 



The tubules depending from the radial blood-vessel and entering the subambulacral 

 plexus gradually become less and less aliundant, and finally disaj^pear 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 Pentacrimis uyville- 

 thomsoni (PI. LVII. fig. 1, Ijj), 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 difliculty 



