r w.v. 



ECHINODERMATA 627 



ing known as the madreporite (see below), a radial vessel along each 

 radius, and lateral branches from the radial vessels to the tube feet, 

 which, when the latter are used for walking, possess swellings known 

 as ampullae ^ by whose contractions the 

 feet are extended; (5) the madreporic 

 vesicle^ an inconspicuous cavity of mor- 

 phological importance (see below); 

 (6) the axial sinus. This is a space which 

 varies greatly in its development. It is 

 conspicuous in the Asteroidea, small in 

 the Echinoidea and Ophiuroidea, very 

 small in the Holothuroidea, merged in 

 the perivisceral cavity in the Crinoidea. 

 It communicates with the exterior (or, as 

 will be seen, in most holothurians with 

 the coelom) by a pore or set of pores 

 situated in one of the interradii. This 

 opening constitutes the madreporite. 

 The stone canal opens into the axial 

 sinus just below the madreporite, and so 

 the latter serves as the opening of the 

 stone canal. In the Asteroidea and 

 Echinoidea the madreporite is a con- 

 spicuous structure on the aboral side, pierced by many pores. In the 

 Ophiuroidea it is on the oral side, and has one pore, or only a few 

 pores. In most of the Holothuroidea it becomes detached, in the 

 course of development, with its tiny axial sinus, from the body wall, 

 and hangs into the perivisceral cavity, with which, instead of with the 

 exterior, it now makes communication, by a number of pores. In this 

 group, by meristic repetition, there may be several or many stone 

 canals, each with an "internal madreporite". In the Crinoidea, the 

 stone canals, of which there are several, end each by a single opening 

 into the perivisceral cavity, and the latter communicates by a number 

 of pores with the exterior. 



In the bilateral larva {Dipleurula) ^ the coelom (Fig. 437) is present 

 as three pairs of sacs, of which the first is preoral. The second pair is 

 connected by a passage with the first: the third is independent. In 

 outline, the relation between these sacs or segments of the larval 

 coelom and the coelomic spaces of the adult is as follows : the peri- 

 visceral cavity of the adult is formed by the fusion of the main portions 

 of the hinder pair ; the aboral sinus system becomes separated from 

 the perivisceral cavity; the perihaemal system arises as outgrowths 

 from the left hinder cavity (in some cases it receives a component also 

 from the left anterior cavity); the water Vascular system ("hydro- 



Fig. 436. A diagram of the 

 water vascular system of a 

 starfish. FromBorradaile.am/). 

 ampulla ; mad. madreporite ; 

 r.w.v. radial water vessel ; st.c. 

 stone canal; ?./.z;. vessel of tube 

 foot; w.z;.r. water vascular ring. 



