444 COMPARATIVE ANATOMY CHAP. 



D. The Perianal Sinus. 



In the Holothurioidea (with the exception of the Synaptida?) and in 

 the Echinoidea the end of the hind-gut is connected with the neigh- 

 bouring body wall by means of a circular membrane, which cuts off 

 a small perianal sinus from the general body cavity. If the circular 

 muscle fibres, with which the walls of this sinus are abundantly sup- 

 plied, contract, they act as sphincters, and close the anus. In the 

 regular Echinoids, below the perianal sinus, there is a second closed 

 sinus surrounding the hind-gut ; this is the periproetal sinus. 



E. The Axial Sinus. 



1. Asteroidea. In the madreporitic interradius the general body 

 cavity is traversed by a large, vertical, flattened tube, with tough, flat, 

 radially arranged lateral walls. This tube connects the region of the 

 madreporite with the ventral body wall. The cavity of the tube is an 

 enclosed portion of the true body cavity, and in a young stage is in 

 open communication with the enterocoel ; it is known as the axial 

 sinus (sac- or tube-like canal, sac hydrophorique). It surrounds and 

 contains (1) the stone canal, which runs down from the madreporitic 

 plate to the circular canal ; and (2) the axial organ (dorsal organ, 

 heart, pseudo-heart), which is attached to its wall by means of a 

 mesentery. The tough wall of the axial sinus consists of the follow- 

 ing layers: (1) the ciliated endothelium of the body cavity (on the 

 side facing that cavity) ; (2) longitudinal muscle fibres ; (3) connective 

 tissue ; (4) the inner ciliated epithelium lining the axial sinus. The 

 axial sinus opens dorsally into the aboral circular canal (circular 

 sinus) of the genital system. 



2. Ophiuroidea (Fig. 361, p. 422). In consequence of the shifting 

 of the madreporite on to the oral side, the stone canal which springs 

 out of the circular canal bends outward and downward. At its distal 

 end it is connected with a small ccelomic sinus, usually called an 

 ampulla, which lies on the side turned to the centre of the disc, and 

 itself opens outward through the water vascular pore. This sinus no 

 doubt corresponds with the axial sinus of Asteroids. Another sinus 

 accompanies the stone canal on the side turned to the periphery of the 

 disc, and opens into the circular canal of the genital system. That 

 portion of the wall of this second canal which is in contact with the 

 stone canal is developed as the ovoid gland. 



3. Eehinoidea (Fig. 358, p. 419). The axial sinus, which runs 

 up from the circular canal to the apex and is accompanied by the 

 stone canal, is here almost completely filled by the large axial 

 organ. It is completely cut off by a septum from a spacious sinus 

 which lies near the ampulla, and into which a process of the axial 

 organ projects. The two communicate only in an early stage of 



