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



the Apoda and Pedata, of course with several exceptions, belong to the shores. The 

 more important peculiarities in the organisation of the Holothurioidea, especially of 

 the Elasipoda, may now be pointed out. 



" It is known that a large majority of the so-called shallow water forms, viz., the Apoda 

 and most of the Dendrochirotse, have a cylindrical or fusiform body, the former destitute 

 of all pedicels and processes, the latter provided with small cylindrical pedicels, either 

 irregularly scattered all over the body or arranged in rows along the ambulacra. 

 Consequently, no clear distinction between the dorsal and ventral surfaces is here marked 

 out. In the rest of the Dendrochirotse and the Aspidochirotae, on the contrary, more or 

 less clearly marked dorsal and ventral surfaces are present, carrying processes or pedicels 

 scattered or in rows. 



" A glance at the figures given in this account, and representing types of the three 

 families into which the Elasipoda are divided, will clearly show that they are characterised 

 by a ventral and dorsal surface, distinctly marked the one from the other, and, in general, 

 by the bilateral symmetry of the body, — characters which they have in common with the 

 Aopidochirotse and part of the Dendrochirotse ; but above all, by the unusual symmetry 

 in the arrangement of the pedicels and processes. The following peculiarities cannot be too 

 clearly expressed as characterising the Elasipoda : — The ambulacral appendages of the 

 ventral surface alone are intended for locomotion, these being in the typical Elasipoda par- 

 ticularly large, and arranged in a single row on each side of the body ; and the locomotor 

 organs of the one side are accurately opposed to those of the other side, so as to form 

 distinct pairs, almost recalling the legs of an insect or the locomotor organs of one 

 of the Polychaeta. As a rule the locomotor organs of the Elasipoda are not to be 

 compared with such true pedicels as are common in other Holothurids, but are rather to 

 be regarded as processes or ' ambulacral papillae.' 



" These locomotor organs show the most evident tendency to appear in fixed places 

 and in a fixed number in every species of the more typical Elasipoda, and their number 

 is often limited, as, for instance, in Elpidia glacialis, which has always four pairs of 

 pedicels, Scotoplanes globosa, which has seven pairs, &c. 



" The dorsal appendages are so modified as to perform functions far different from 

 those of the ventral appendages. These dorsal appendages, like the ventral ones, have 

 a tendency to become definite in number, so that every species may have a certain 

 number situated in certain fixed positions on the back. 



" From the size of the pedicels and their incapability of extension, and from the fact 

 that the pedicels mostly lack a terminal plate, and sometimes even a sucking disk, the 

 Elasipoda seem to be unable to move in the same manner as most of the Echinoderms, 

 by attaching the suckers to surrounding bodies. Besides, their often firm external 

 skeleton, and the shortness of their body-form, probably prevent them from moving by 

 the extension and contraction of their bodies. From the size of the pedicels and their 



