ASTEROIDEA. 181 



made up of as many interradial pieces as there are arms, each 

 piece being prolonged into two adjacent arms. It thus comes 

 about that each arm has a prolongation of two pieces of the 

 sa-called outer perihaemal ring, the septum between these two 

 radial prolongations being the septum (Fig. 129, 3) which divides 

 the radial perihaemal canal into two parts and in which the 

 so-called radial blood-vessel runs. This outer circumoral peri- 

 haemal ring is derived for the most part from the left posterior 

 body-cavity of the larva (see p. 145). It is lined by a flat 

 epithelium, whether ciliated or not is unknown. 



There is said to be a canal in the body-wall at the edge of the ambulacra! 

 groove which communicates at intervals with the radial perihaemal space 

 and with the perivisceral cavity (Fig. 1'29, 19). 



The aboral sinus (Figs. 131and 132a&) is a circular or penta- 

 gonal sinus placed on the aboral side of the stomach between 

 it and the skin, and giving off in each interradius two prolonga- 

 tions * to the generative organs (gen.r). In this sinus and its 

 prolongations lies a peculiar cord of tissue, consisting partly of 

 generative rachis and partly of vascular tissue. The space, formerly 

 supposed to be a blood-vessel, in the wall of the gonads, is de- 

 veloped as a part of this system, but in the adult there is a 

 septum which shuts it off from the rest. The whole of the 

 aboral sinus with its prolongation to the gonads and the sinus 

 in the walls of the latter is developed as a part of the left pos- 

 terior body cavity of the larva. 



The water-vascular system is lined throughout by a ciliated 

 epithelium, has thin muscular walls, and contains a colourless 

 albuminous fluid in which float amoeboid cells. It consists of 

 a circumoral vessel, which is placed on the inner side of the 

 buccal membrane close to the calcareous pieces of the peristom, 

 sends prolongations the radial vessels into the radii, and 

 communicates with the exterior through the stone-canal and 

 axial sinus. The stone-canal passes aboralwards in interradius 

 //. /// (the so-called left anterior). At its aboral end it opens 

 into the axial sinus on the one hand and to the exterior through 

 the pores of the madreporite on the other. The madreporite is 



* The two cords which pass from the point where the axial sinus abuts 

 upon the aboral sinus to the stomach are not processes of the generative 

 rachis and aboral sinus as was formerly supposed, but of the wall of the 

 axial sinus (see p. 177). 



