ECHINODERMATA 



55- 



mad 



known as the stone canal because its wall is frequently calcified, lead- 

 ing to an opening 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 swell- 

 ings known as ampullae, by whose con- 

 tractions the feet are extended; (5) the 

 madreporic vesicle, an inconspicuous 

 cavity of morphological importance (see 

 below); (6) the axial sinus. This is a 

 space which varies greatly in its develop- 

 ment. It is conspicuous in the Aste- 

 roidea, small in the Echinoidea and 

 Ophiuroidea, very small in the Holo- 

 thuroidea, merged in the perivisceral 

 cavity in the Crinoidea. It communi- 

 cates 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 conspicuous 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. 412) 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 out- 

 line,the relation between these sacs orsegments of the larval coelom and 

 the coelomic spaces of the adult is as follows : the perivisceral 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 



Fig. 411. A diagram of the 

 water vascular system of a 

 starfish. FromBorradaile. amp. 

 ampulla ; mad. madreporite ; 

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

 stonecanal; ^./.z;. vessel of tube 

 foot ; iv.v.r. water vascular ring. 



