348 ECUINODERMA. 



lacra alone are locomotor, those of the bivium being tactile or 

 wholly absent. 



In the body cavity (fig. 336) lies the alimentary canal, which 

 (except in Synapta) is coiled in a uniform manner, although many 

 minor convolutions may obscure this. It passes backwards in the 

 median dorsal interradius,forwardin the left ventral interradius, and 

 then back in the right dorsal interradius to the anus. It is held in 



position by mesenteries (fig. 337), and 

 near the anus by numerous muscular 

 filaments. Into the terminal portion 

 one or two branchial trees may empty. 

 These are tubular sacs with small 

 branched outgrowths which are filled 

 with water. The similarity of these to 

 the excretory organs of some Gephy- 

 raea (p. 317) was one ground for re- 

 FIG. 337. Transverse section of garding those forms as intermediate 



Hnlothuria tubulosa. (After Lud- f 



wig.) d, digestive tract ; db, dor- between worms ana ecnmoderms. 



sal blood-vessel; 0, gonad duct; ,, n n 



n, skin; tm, longitudinal muscles; They are to be regarded as respiratory, 



liv, left branchial tree ; wi, mesen- i-i j* n .en j -j.i 



teries ; r', r 3 , ambuiacrai complex since they are periodically mled with 



f resh water. In many species < Cuvie- 



, right branchial tree. rian organg , OC(Jur . thege are morpho . 



logically specially modified portions of the branchial tree and are 

 either connected with them or separately with the cloaca. Many 

 zoologists regard them as defensive structures because of their 

 sticky nature and because they can be cast out through the anus. 

 The oesophagus is usually surrounded by a ring of five radial 

 and five interradial plates which serve as points of attachment for 

 the longitudinal muscles. Just behind it lie the ring canal, ring 

 nerve, and the ring of the blood system, each giving off a radial 

 branch which here runs inside the muscular sac of the body. From 

 the beginning of the radial canals (rarely, as in Synapta, from the 

 ring canal) tubes extend outward to form the extremely sensitive 

 retractile tentacles which surround the mouth, and which either 

 branch (Dendrochirotae) or bear frilled shield-shaped extremities 

 (Aspidochirotae). A single Polian vesicle is usually present, and 

 the stone canal (except in the Elasipoda) connects with the 

 coelom. Blood-vessels going from the vascular ring form rich 

 anastomoses on the alimentary canal. Only a single gonad (or a 

 pair of united gonads) occurs. This consists of numerous tubules 

 which open usually interradially near the mouth. 



