REPORT ON THE HOLOTHURIOIDEA. 131 



of the ventral surface. I counted eighteen to twenty attached to the anterior half of 

 the nerve-stems, to which they seem to be connected either directly or by means of 

 a short branch. 



In Peniagone vitrea and Peniagone affinis a greater number of vesicles are to be 

 observed along both of the ambulacral nerves just mentioned. 



There is no doubt that the majority of the ELpidiidae are characterised by possessing 

 auditory vesicles, and that these are generally disposed in the same manner as in 

 Elpidia glacialis and Kolga hyalina. Their number as well as that of the otoliths seems 

 to vary very considerably, the latter being sometimes, though in a very few cases, alto- 

 gether wanting. The otoliths in all the species I have seen are distinguished by the same 

 characteristic ovate shape, with one end rounded and the other tapering and truncated. 



The Alimentary Canal. 



With the exception of a few Synaptidse, in which it takes a straight antero-posterior 

 course, the digestive tract of the Pedata and Apoda, as well as of the Elasipoda, is of 

 a considerable length and arranged in convolutions. It descends from the mouth to 

 the opposite extremity of the body, where, turning upon itself, it mounts up towards 

 its anterior portion, whence, turning back again, it once more passes backwards directly 

 to the anus (PI. XL. fig. 2). Thus one can distinguish the following portions of the 

 digestive tract, one descending, another ascending, and a third again descending. 

 As previously pointed out the mouth as well as the anus change their positions in the 

 Elasipoda ; the former always has a more or less ventral inclination and is often directed 

 straight downwards, and has a perfectly ventral aspect in the whole of the Psychropotidae, 

 while the latter is alternately ventral, terminal, and dorsal. The oral aperture, which is 

 closed by a sphincter formed of numerous circular muscular fibres, is situated in the 

 centre of the oral disk, which, surrounded by the tentacles, is only to be regarded as a 

 part of the body-wall itself. The space between the tentacles and the mouth is termed 

 the atrium. As is already known, in the Dendrochirotse the oral end of the body bears 

 a certain resemblance to a proboscis which is capable of retraction, a capability depending 

 upon the thinness of the integument of the proboscis itself. In the Elasipoda the oral 

 disk is in general thick and almost inflexible because of the more or less crowded 

 calcareous bodies in it, consequently it is not retractile (PI. XLIII. fig. 1). But excep- 

 tions have been found, and particularly in the genus Deima, where the oral disk seems 

 to be more or less allied to that of the Dendrochirotae, and deserves therefore to be de- 

 scribed more in detail. 



In all the representatives of the genus Deima which have been at my disposal, no 

 tentacles have been apparent externally. One finds in the centre of a disk-like portion 

 of the body-wall an aperture, which is closed by large circular muscular fibres (PI. XLIII. 

 figs. 2, 3.) The disk in question is supplied with a number of radiate wrinkles, 



