REPORT ON THE HOLOTHURIOIDEA. 147 



and there is no doubt, indeed, that the Elasipoda as well as the Pedata approach much 

 nearer in this respect than the Apoda to the typical Echinoderms. I cannot conceive 

 how the fact that the Elasipoda have retained more larval characteristics than the Apoda 

 can in any way subvert this opinion. Lastly, concerning the mutual position of the 

 Elasipoda and the Pedata, the former certainly are in one respect more nearly related to 

 the typical Echinoderms than are the latter, viz., by their madreporic canal often com- 

 municating with the exterior, and that too not only by one pore but sometimes by a great 

 number of pores crowded close together so as to form a kind of exterior " madreporic 

 tubercle," but this peculiarity alone does not seem to me to decide their relative position. 

 No doubt, the Pedata have, on the other hand, many important characteristics which may 

 entitle them to a higher place in the series of Echinoderms. 



But disregarding their position as Echinoderms, and considering only their general 

 development as animals, I cannot but think that the Elasipoda have already in certain 

 respects attained a higher development than all the other Echinoderms, and that this 

 development is gradually advancing in a direction approaching the higher classes of 

 animals. This opinion is founded on the following reasons : — 



1. The form of the body is distinctly bdaterally symmetrical, with the ventral and 

 dorsal surfaces clearly distinguishable from each other, with the mouth on the ventral 

 surface and often with a head-portion plainly marked off from the rest of the body. 



2. The ambulacra! appendages of the ventral surface alone are intended for locomo- 

 tion, these being in the typical Elasipoda particularly large and arranged on each side 

 of the body in a single row ; and the locomotive 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 locomotory organs of one of the Polychceta, &c. 



3. These locomotory 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 that their 

 number is often rather limited, as, for instance, in Elpidia glacialis, which has always 

 four pairs of pedicels, Scotoplanes globosa, which has five pairs, &c. 



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

 those of the ventral appendages. 



5. These dorsal appendages, like the ventral ones, have a tendency to become fixed 

 in number so that every species may have a certain number situated in a certain place 

 on the back. 



VVO ( 



