562 ECniNODERMATA HOLOTHUROIDEA chap. 



in order to distinguish them from the pointed podia scattered 

 over the biviura. This procedure will be adopted here. In the 

 Cotton-spinner the feelers, when extended, show a short smooth 

 stem, from the apex of which springs a circle of short branches, 

 which are in turn beset with a double row of branchlets, 

 themselves branched. Such feelers are said to be sliield-shaped. 



A transverse section through the radius of a Sea-cucumber 

 is, in general, like one through the radius of a Sea-urchin ; the 

 points of difference to be noted are : (a) In the Sea-cucumber, 

 beneath the ectoderm, is a thick dermis with small j)lates 

 scattered in it, instead of the whole dermis being calcified, as is 

 the case in the Sea-urchin ; (h) the ampulla of each podium is 

 connected with tlie peripheral portion by one canal, not two, as 

 in the case of the Sea-urcliin ; (c) there is a development of 

 coelomic nervous tissue from the outer side of the perihaemal 

 canal ; (d) internal to the radial water-vascular canal are to be 

 seen cross-sections of two great bands of longitudinal muscles, by 

 the contraction of which the body is shortened. Lengthening is 

 brouglit about by the contraction of transverse muscles, which 

 are found on the inner side of the body -wall in each inter- 

 radius ; the five sets taken together act like circular muscles, or 

 a rubber band, on the incompressible fluid in the body-cavity. 



When the Sea-cucumber is opened by a cut along the left 

 dorsal interradius, the spacious coelom is laid open, and lying in 

 it is seen the alimentary canal. This tube is bent on itself, so 

 that it has a form like co (Fig. 255, B) running backwards to the 

 posterior end of the body, tlien running forwards to near the 

 anterior end, before it finally turns to run backwards to the 

 anus. By taking cross-sections of the body at different levels, it 

 can be shown that the alimentary canal makes a half-turn round 

 the longitudinal axis (Fig. 255, A). It is suspended b}^ bands of 

 membrane, termed " mesenteries," to tlie body-wall, and of these 

 there are three, the first of which (i.e. the one nearest the 

 mouth) is attaclied to the mid-dorsal interradius (Fig. 255, A, M^}, 

 the next to the left dorsal interradius (M^), and the last to the 

 right ventral interradius (M^). 



The alimentary tube shows four regions, whicli are distin- 

 guished as follows: — (1) A short oesophagus with strongly- 

 marked longitudinal folds in its walls ; this is separated by a 

 constriction from (2) the stomach, a very short region, character- 



