CLASSIFICATION 569 



are few, long, and stiff, and often coalescent in places to form 

 grotesque or remarkable appendages. In the remaining forms 

 the podia of the trivium have always suckers, whilst those of 

 the bivium may or may not be pointed. In Psolus the two 

 dorsal radial canals and their podia are totally absent. 



Respiratory Trees. These are present in Aspidochirota, 

 Dendrochirota, and Molpadiida, totally absent in the Synaptida 

 and Pelagothuria, and doubtfully represented in a few Elasipoda 

 by a single unbranched outgrowth of the gut. 



Skeleton. This consists, as explained above, of the scat- 

 tered deposits in the skin and of the calcareous ring. As 

 regards the first, their shape varies immensely, and yet one or 

 two principal types characteristic of each of the main divisions can 

 be defined. Thus the Synaptida are characterised by wheels, 

 with spokes ending in a hub, and by anchors attached to a plate. 

 The Elasipoda have simple St. Andrew's crosses, whilst the 

 Aspidochirota are mainly characterised by "stools" (Fig. 257, C) 

 and buckles (Fig. 257, D). The Dendrochirota have a bewilder- 

 ing variety of forms ; the most characteristic, however, are 

 a right-angled cross and a grating, very similar to the buckles 

 of the Aspidochirota, except that in the former there are usually 

 four holes placed cross-wise, whilst the buckle has generally two 

 parallel rows of three holes. Since these ossicles are the only 

 records we possess of the existence of fossil Holothuroidea, they 

 have been studied with great care. The calcareous ring varies 

 very much. The radials are always five (except in individuals 

 where there are more than five radii), but the interradials are 

 increased in the Synaptida, and in the other orders are in some 

 cases diminished or occasionally suppressed altogether. The 

 last is the case in nearly all Elasipoda ; here the radials consist 

 of a central horizontal piece with two diverging arms at each 

 side. These arms, which can branch repeatedly, traverse the 

 adjacent interradii, meeting those of the next radii, so that inter- 

 radials are in most cases entirely absent. The Aspidochirota have 

 usually a ring consisting of small squarish ossicles (Fig. 257, B). 

 In the Molpadiida and Dendrochirota the radials are prolonged 

 backwards into forked tails, which in some Dendrochirota are 

 broken into a number of small pieces (Fig. 257, A), the lower 

 parts of the interradialia being similarly divided. 



The classification of the Holothuroidea is comparatively easy. 



