TYPE ECHINODERMA. 587 



may be considerably increased. Five interradial canals arise 

 from the ring and pass forwards to the tentacles, branching if 

 these structures are more than five, and in some forms {Holo- 

 thuria, Ghirodota) these tentacular canals are provided with 

 ampullae. Five radial canals also pass backwards from the 

 ring in all forms except the Synaptidse, corresponding to the 

 radial hydrocoel-canals of other Echinoderms and bearing 

 tube-feet. 



The distribution of these latter structures is peculiar in 

 many forms. In Molpadia although the canals are present 

 the tube-feet are entirely wanting, as they are also from the 

 dorsal canals of Psolus and from the median canal of the 

 trivium of the Elasipoda ; when present they may be arranged 

 along the lines of the radial canals {Gucumaria, Pentacta, Fig. 

 268) or may be scattered irregularly over the surface of the 

 body (Thyone). In form they also vary considerably, being 

 either simple fingerlike processes or else tipped with a 

 sucker. Frequently the tube-feet are not retractile, and in 

 the Elasipoda they take the form of strong well-developed 

 conical processes arranged in pairs. 



Owing to the absence of a firm test in the Holothurians 

 there is a much more extensive development of the muscular 

 system than in other Echinoderms. The inner surface of the 

 body-wall is formed by a layer of circular muscle-fibres, and 

 on each side of each radial hydrocoel-canal is a longitudinal 

 muscle-bundle (Fig. 269,^) from which in some forms special 

 bundles pass to the peripharyngeal ossicles and serve as 

 retractors of the tentacles and mouth-disk. 



As stated the mouth is usually at the anterior end of the 

 body at the centre of a disk surrounded by the tentacles, but 

 in the Elasipoda it has a somewhat ventral position. The 

 digestive tract is a simple tube, which occasionally has a per- 

 fectly straight course from mouth to anus, but more frequently 

 it is bent twice upon itself, so that there is an anterior de- 

 scending limb (Fig. 269,/), an ascending (g), and a posterior 

 descending one (h). The terminal portion of the posterior de- 

 scending limb is dilated, forming a cloaca (i) from whose wall 

 muscle-bands (q) radiate to the walls of the body. This cloaca 

 is rhythmically contractile, and has opening into it except in 



