REPORT ON THE HOLOTHURIOIDEA. 113 



are very remarkable on account of their great flatness. In other forms as, for instance, 

 Scotophilia insignis, Elpidia willemoesi, &c, the body decreases in height backwards, 

 its posterior portion becoming almost flat, while the contrary may be observed in the 

 genus Psgchropotes, which is remarkable for the height of its posterior and the flatness 

 of its anterior extremity. In several species, as in Scotoplanes robusta, Peniagone 

 vitrea, Elpidia willemo'esi, &c, the body is produced anteriorly into a narrow, longer 

 or shorter neck-like part, which is directed straight downwards. 



Another peculiarity which characterises the external appearance in a great number of 

 Elasipoda is the presence of a brim which surrounds either the posterior extremity of the 

 body as in Scotoplanes insignis, Elpidia -willemoesi, &c, or both the posterior and the 

 anterior extremities, as in Scotoanassa diaphana, or only the anterior one, as in Elpidia 

 purpurea and Enypniastes eximia, or which, finally, extends round the whole body, 

 as in the Psychropotidae, where it sharply defines the dorsal from the ventral surface. 

 The brim in question, which originates in an enlargement of the layer of connective 

 tissue of the perisoma, is broad and rather thin, and the wide tubes which penetrate it and 

 unite the pedicels with the ambulacra often become visible externally, thus giving the 

 brim a somewhat fin-like appearance. Sometimes, as in Scotoplanes insignis and Elpidia 

 willemoesi, &c, this fin-like brim becomes bipartite at the posterior extremity. The 

 Psychropotidae are easily distinguished by the possession of this brim, which surrounds 

 the body and attains such a breadth at the anterior end as to give it the appearance of 

 a more or less distinct head. 



The two orders Apoda and Pedata sometimes have the mouth and anus almost 

 terminal at opposite poles of the cylindrical or fusiform body, sometimes on the contrary 

 directed upwards, as in several Dendrochirotae remarkable for their ascidia-shaped body. 

 In a great number of Aspidoehirotae the mouth and the oral disc are more or less 

 distinctly turned towards the ventral surface. 



In the Elasipoda the mouth and the surrounding oral disk, though situated either at 

 or near the anterior end of the body, are always more or less evidently directed towards 

 the ventral surface ; in many forms, indeed, the mouth lies in the same plane as the 

 ventral surface. Even in forms characterised by a body of cylindrical shape, as, for 

 instance, Parelpidia, the oral disk has a slight though definite ventral inclination. 

 Again, one cannot invariably describe the mouth as terminal, because in numerous 

 forms such as Elpidia purpurea, and in all the Psychropotidae the dorsum projects 

 considerably beyond the mouth. Now, the under surface of this projection lies in the 

 same plane as the ventral surface or trivium, and thus the mouth seems to be placed on 

 the ventral surface at some distance from the anterior extremity. 



Here, then, it is evident that the dorsum projecting beyond the mouth constitutes in 

 a measure the anterior end of the body, but it is of importance to remember that the 

 peritoneal cavity does not extend beyond the mouth, or, at most, very slightly, so that 



(ZOOL. CHALL. EXP. — PAET XIII. — 1881.) N 15 



