REPORT ON THE HOLOTHURIOIDEA. 135 



essentially from those of the Peclata. There is always a circular canal which surrounds 

 the gullet and sends forwards five " main canals " (PI. XLIII. fig. 4, k), which 

 divide into branches communicating with the tentacles, and give off five radial 

 ambulacral vessels which run backwards along the longitudinal muscular bands, 

 separating them from the ambulacral nerves. In my report on the genus Elpidia l 

 I showed that this genus had only two radial ambulacral vessels, one along each side 

 of the ventral surface. After that, Danielssen and Koren not only established the 

 truth of my observations concerning Elpidia, but also asserted that their own new 

 genera Irpa and Kolga had only two radial vessels. Having most carefully examined 

 the rich material of partly enormous forms brought home by the Challenger Expedition, 

 I am fully convinced that all the Elasipoda are provided with five radial ambulacral 

 vessels. By injecting the odd ambulacral vessel, which is easily done in Lcetmogone, 

 Oneirophanta, Scotoplanes globosa, and in many other large forms, not only does 

 the vessel in question become filled with colouring matter, but also the circular canal 

 and its branches, and the same result is attained by filling the dorsal ambulacral vessels 

 or their processes with some coloured liquid. Even by means of transverse sections of 

 these ambulacra one can easily be convinced that these vessels are present in all forms 

 which I have examined, and, consequently, even in Elpidia glacialis and Kolga nana 

 (PI. XLII. fig. 8). It must be admitted that I have had no opportunity to examine 

 Irpa abyssicola and Kolga hyalina described by Danielssen and Koren, but it would 

 be very singular if such closely allied forms should form exceptions to the rule. 



Danielssen and Koren 2 are right in not agreeing with the opinion expressed by me 

 concerning the lateral ventral ambulacral vessels of Elpidia glacialis. In fact, I 

 succeeded no better in distinguishing the true lateral ventral vessels than the other vessels, 

 for what I described as such are only the large very remarkable ambulacral vesicles 

 or rather cavities which communicate with the pedicels, lie side by side and are closely 

 united with one another, thus appearing like a large wide canal which runs along each 

 side of the body and is divided by very thin double walls into cavities, corresponding 

 to the pedicels in number. These partition walls were most strictly described by me, 

 and the figures 4, 28 and 29 in my memoir on the Elpidia give a true idea of their 

 appearance. The extremely narrow and fine ambulacral vessels which become dis- 

 tinguishable only in transverse sections, had escaped my attention. Thus, it is evident 

 that the general arrangement of the water-vascular system in the Elasipoda corresponds 

 to that in the Pedata. 



The circular water- vessel, which in some forms, as, for instance, in Oneirophanta, 

 Deima, Orphnurgus, &c, is very wide, but in others, as, for instance, in Benthodytes 

 sanguinolenta, &c, seems to be comparatively much narrower, does not closely 



1 Kongl. Svenska. Vetenskaps Akademiens Handlingar, Bd. xiv. No. 8, Stockholm, 1877. 



2 Eckinodermen fra den Norske Nordhavsexpedition (Nyt Magazin for Naturv., Bd. xxv. 2, Christiania, 1879). 



