viii ECHINODERMATA WATER VASCULAR SYSTEM 417 



Calcareous corpuscles may be formed in the connective tissue layer of the wall in 

 other parts of the water vascular system besides the stone canal. Such calcification 

 always takes place in locomotory tube-feet. 



The fluid contained in the water vascular system is sea-water 

 with traces of albumen (in a '5 - 2 per cent solution). Floating in 

 this fluid are found amoeboid cells (lymph bodies) and coloured cor- 

 puscles often united into small lumps. The fluid occasionally appears 

 of a pale yellow, or reddish, colour. 



The origin of this fluid is a question of frequent recurrence. 

 The view which still appears best supported is that sea-water flows in 

 through the madreporite and the stone canal, but an exactly opposite 

 view has also been maintained. The observations made on this 

 subject appear to contradict one another, it being very difficult to 

 carry on investigations in a decisive and satisfactory manner. 



A. Madreporite and Stone Canal. 



1. Holothurioidea (Fig. 357). The condition which must be con- 

 sidered as the original is that in which only one stone canal occurs ; 

 this is attached to the dorsal mesentery (cf. p. 406), and its 

 madreporite lies mediodorsally in the integument, and its pore 

 canal or canals open outward direct. 



Such a condition is found in the adult only in certain Elasipoda 

 and in Pelagothuria. 



In the large majority of Holothurioidea, the stone canal loses all 

 direct communication with the exterior, while at its distal end, 

 which now lies in the body cavity, a new inner madreporite forms, 

 through whose canals communication is established between the 

 stone canals and the body cavity. 



In a comparatively small number of Holothurioidea (never in 

 Molpadiidce and JElasipoda) the number of stone canals increases (the 

 single canals usually shortening at the same time), and may finally 

 become very great (over 160). 



The inner madreporite is found in the form of a variously shaped swelling on 

 the stone canal, which is often S-shaped or spirally coiled. Only the primary stone 

 canal is connected with the dorsal mesentery ; this is never the case with accessory 

 canals. These latter float freely in the body cavity, and this is also the condition 

 of the primary stone canal of the Aspidochirotce, which has lost its connection with 

 both the body wall and the mesentery. 



More than one canal is found in only a very small number of forms even among 

 the Synaptidce, the Dendrochirotce, and Aspidochirotce. The number of accessory 

 canals varies greatly in different forms ; it does not seem to be of systematic import- 

 ance, since it varies in individuals of one and the same species. It is probable that 

 the accessory canals, ontogenetically, bud off* secondarily from the water vascular 

 system, whereas the dorsomedian stone canal arises primarily from the canal which, 

 in the larva, connects the hydroccel with the exterior. 



Branched stone canals, with a madreporite at the distal end of each branch, 

 occur in Synapta Beselii, Jag ; and Thyone chilensis, Semp. 



VOL. II 2 E 



