WATEE- VESSELS OF ECHINODERMA. 



223 



these parts have a different relation to that which they have in the 

 rest of the Echinoderma. The walls of the stone-canal, which 

 hangs freely into the coelom, are more or less calcified; when they 

 are more so they form a firm capsule. The porous parts of the 

 canal are usually distinguished by calcification, and so repeat within 

 the body the arrangements of a madreporic plate. The ends of 

 each branch carry a porous 

 piece when the stone-canal 

 is broken up into branches ; 

 this repetition of parts leads 

 to the formation of race- 

 mose structures, which are 

 only functionally similar to 

 a number of madreporic 

 plates grouped around the 

 stone-canal. The stone- 

 canals vary in number, as 

 well as in arrangement. 

 Often only one is present ; 

 in other cases, and notably 

 in the Synaptas, there are 

 several arranged around 

 the circular canal. The 

 number too of the Polian 

 vesicles (Fig. 113, p), which 

 are present in these forms, 

 varies; in Holothuria and 

 Molpadia there is one, in 

 Synapta Beselii about fifty, 

 and in Cladolabes about a 

 hundred. 



The canals from the 

 circular canal (0) run for- 

 wards inside the calca- 

 reous ring (ii), and give off 

 branches to the oral tenta- 

 cles (T) ; a cascal elongated 

 tube, corresponding to the 

 ampullas of the suckers, is 

 connected with each of them, These tubes are of some size in 

 the Holothurida, and lie on the outer side of the calcareous rine- : 

 they are only feebly developed in the Synaptidse. The radial trunks 

 going to the ambulacra are placed, in Holothuria, in the bundles 

 of longitudinal muscles, which are thus divided into two halves. In 

 Cucumaria they are placed on the outer side of these muscles. The 

 branches of these canals, as in other forms, go to the feet. When 

 the feet are atrophied the vascular branches, which go to them, are 

 atrophied also ; but the principal trunks appear to persist even in 

 the Apodia, for they have been observed in Synapta, although, 

 indeed, diminished in size. 



Fig. 113 Longitudinal section through the 

 anterior part of the body of Synapta digitata. 

 R R' Calcareous ring, r Muscles passing from it 

 to the oesophagus, o Mouth. D Enteric tube. 

 C Circular canal, t "Canals to the tentacles T. 

 p Polian vesicles, n Nerve-ring, n 1 Eadial 

 nerve-trunk, passing through the calcareous ring 

 R 1 . m Bands of longitudinal muscles. G Ducts 

 of the generative organs (after Baur). 



