130 PHYLUM ECHINODERMATA. 



young larva, but later becomes merged in the general perivisceral 

 cavity, so that in the adult both the stone-canals and the water 

 pores open into the perivisceral cavity. In Holothurians the 

 anterior coelom is very feebly developed, but as Bury has shown 

 it can be traced through the whole development (p. 152) and 

 has the appearance of a small appendage of the stone-canal 

 (Fig. 108). In a few Holothurians it retains its communication 

 (by the water-pore) with the exterior, but in the majority the 

 water-pore closes in the adult, and the walls of the anterior 

 body-cavity largely break down and give rise to the so-called 

 internal madreporite. It thus comes about that in most Holo- 

 thurians the stone-canal appears to open directly into the general 

 body cavity. 



It sometimes happens (some Asteroids and Ophiuroids, Crinoids) that 

 there is more than one primary water-pore ; in such cases there is a cor- 

 responding increase in the number of madreporites and stone-canals, and 

 they are generally placed in different interradii. In Crinoids there are 

 never less than five primary water-pores, and generally they are much 

 more numerous. In the former case there is one water-pore and one 

 stone-canal in each interradixis ; in the latter case there are many of both 

 structures in all the interradii, and an exact correspondence between the 

 stone-canals and water-pores, though it may exist, cannot be shown. 



The water- vascular system is lined by a flat ciliated epithelium 

 and contains an albuminous fluid with leucocytes very similar 

 to that found in the perivisceral cavity and sinus system. Its 

 function is mainly locomotory, but to this must be added, 

 especially in forms in which the tube-feet are without ampullae, 

 a tactile and respiratory function. By some authors it has 

 been regarded as in part excretory, but there seems to be little 

 evidence in support of this. There might be something to be 

 said for this view, if the current through the madreporite set 

 outward, but the reverse appears to be the case (Ludwig). If 

 there is any current through the water-pore, it appears to be an 

 inward one as a result of which sea-water is drawn into the 

 system. But it may be that on occasion the direction of this 

 current is reversed. Particles of carmine injected into the 

 water-vascular system are taken up by the cells of Tiedemann's 

 bodies (Kowalevsky). 



The axial organ, which goes by various names, e.g. dorsal 

 organ, ovoid gland, heart, etc., is found in Echinoids, Asteroids 

 (Fig. 130), Ophiuroids and Crinoids. It is developed as a fold 



