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



and is surrounded by a simple crown of very minute papilla?, the importance of which 

 will be discussed under the rubric " ambulacral system." By means of this aperture 

 the external medium communicates with a spacious cavity, inside which the small 

 completely retractile tentacles are situated, disposed near and around the aperture. It 

 is most probable that the tentacles can be extended outside the body. In the retracted 

 condition of the tentacles and the surrounding perisoma, no traces of any oral disk 

 corresponding to that in the rest of Elasipoda have been found, unless the insignificant 

 circular fold which lies close behind the tentacles may be considered as such. Even in a 

 few other cases, as, for instance, in Benthodytes selenkiana, &c., the crown of tentacles 

 can be drawn within the body, but this seems to take place in the same manner as in 

 the Aspidochirotse, the oral end of the body not being transformed into a proboscis. 



The most anterior portion of the alimentary canal is usually dilated into a cavity 

 (PL XLIII. fig. 1), which is termed the oral cavity, and which extending backwards close 

 to the circular water-vessel is attached by numerous fine threads to the calcareous 

 ring, to the tentacular canals, &c. ; behind the oral cavity a narrower, usually very 

 short, sometimes red or violet portion is to be observed, termed the pharynx, which 

 is separated by a minute circular fold or valve (PI. XLIII. fig. 1, and PI. XXXVI. 

 fig. 1) from the long and wide intestine, which, forming the convolutions spoken of 

 before, terminates in the anus, which is closed by a muscular sphincter. The pharynx, 

 which is surrounded anteriorly by the circular water- vessel (PI. XLIII. fig. 1), and 

 the circular pseudhamial vessels, is connected with these by numerous strong threads. 

 The cloaca, or the most posterior dilated portion of the intestine being attached to the 

 body-wall by numerous powerful bands and threads is either of very little importance, 

 as in the Deimatidse and in several forms of the two other families, or it is, on the 

 contrary, well developed and of an unusual size, as is the case in Elpidia verrucosa, 

 Scotoplanes robusta, Euphronides depressa, &c. Some of the representatives of the 

 Elpidiidse are characterised not only by having an uncommonly large cloaca, but 

 particularly by the fact that the latter gives off a large, wide ceecal prolongation 

 towards the left side, which sometimes extends forwards towards the middle of the 

 body, attaining, at the same time, a very remarkable breadth ; such a cloaca is 

 present in the following species : — Achlyonice paradoxa, Scotoplanes globosa, Scotoplanes 

 albida, Elpidia glacialis, Kolga nana, and Kolga hyalina, Dan. and Kor. Some- 

 times the cloaca seems to vary in size in different individuals of the same species, 

 as in Benthodytes abyssicola, &c. In Deima fastosum the cloaca is separated from 

 the intestine itself by a small circular fold. In Oneirophanta and Deima the hind- 

 most or anal portion of the digestive tract is noticeable in that it can be extended 

 beyond the anus which thus becomes surrounded by a circular mound (PI. XL. 

 fig. 3). By a careful investigation one will find that this anal portion consists of 

 a highly muscular double-wall including a large circular cavity, which, as far as I 



