134 THE VOYAGE OF H.M.S. CHALLENGER. 



mirabile which proceeds from the dorsal vessel in the Aspidochirotse and Molpadidse, and 

 comes into relation with the left respiratory tree. In immediate relation to and seemingly 

 grown together with the water-vascular ring is the annular plexus of the pseudhsemal 

 vessels, which communicates by a larger branch with the reproductive organ. By means 

 of injections I have not only succeeded in distinguishing most plainly the large main- 

 vessels and their branches which are often most conspicuous in Oneirophanta, &c. (PI. 

 XL. fig. 2), but also a delicate plexus of very fine vessels inside the walls of the 

 digestive tract (PI. XL. fig. 1 and PI. XXXVI. figs. 1, 2). 



One or more transverse commissural vessels may always be found. Oneirophanta 

 mutabilis possesses two such vessels, which connect the first descending and the 

 ascending portions of the ventral main vessel with one another ; one of these commis- 

 sural vessels (PI. XL. fig. 2, e) is comparatively long and crosses the other which is much 

 shorter (PI. XL. fig. 2,f), and which divides at both ends into several minute branches. 

 In Deima validum the dorsal as well as the ventral stems carry commissural branches, 

 the former one, and the latter two. The dorsal branch connects the first descending 

 portion of the dorsal main vessel, — with which it is in communication by numerous 

 small branches, — with the ascending portion of the same vessel. One of the ventral 

 commissural branches runs out from the ventral vessel not far from the annular plexus 

 and extends to the ascending portion of this vessel, while the shorter and thicker branch, 

 proceeding a little behind the joint of the former, passes into the anterior curve of the 

 ventral main vessel. Judging from what I have been able to observe in numerous 

 species, a great variety seems to exist with regard to the size and position of these 

 commissural vessels. The walls of the pseudhaemal system being often strengthened 

 by numerous calcareous bodies of varying shape, the vessels themselves often become 

 hard and brittle, as, for instance, in Deima fastosum ; in this species these deposits have 

 the form of large, close-lying, perforated plates of a highly variable appearance, Avhich 

 attain a length of from 0'60 mm. to 070 mm. (PL XXXV. figs. 7, 8). In Lcetmogone 

 wyville-thornsoni, &c, the vessels are on the contrary supported by scattered, spinose, 

 branched or simple spicules (PI. XXXVI. fig. 3). 



The Ambulacral System. 



The general presence of a more or less complicated system of ambulacral vessels is one 

 of the most marked peculiarities of the Echinoderm type, and every contribution to the 

 knowledge of that system of vessels ought to be of the greatest interest and value. The 

 deep-sea Holothurids present an abundance of peculiarities, and their water-vascular 

 system, remarkable in more than one respect, departs in many ways from what is supposed 

 to characterise this class of animals. 



The general characters of the water-vascular system of the Elasipoda do not differ 



