25 



large, strongly convex, and of a more or less circular form. The surface is as usual very 

 uneven, having numerous irregular narrow raised ribs, which radiate from a medial depres- 

 sion ; but there is here in our present species not any trace of spines as in the Brisinga 

 endecacnemos. From the madreporic body, the so-called stone canal (Tab. II, fig. 12 e) 

 extends somewhat obliquely downwards along the interior wall of the bucal ring connecting 

 itself with the circular ambulacral vessel (f). This so-called stone-canal is rather thin, but 

 of a very firm consistency, on account of the numerous porous calcareous particles contained 

 in it; its upper extremity is in great part covered by the wide cuticular sheath (c) which 

 projects into the cavity of the disc and serves to envelope a very problematical organ, the 

 so-called Heart, while its lower extremity lies quite free on one side of the sheath, and con- 

 nects itself with the circular ambulacral vessel, after forming a small S-like curve. The 

 water sucked in by the madreporic plate is still further filtered through the stone canal, 

 and then conducted over to the annular vessel to be conveyed thence further into the radial 

 water-vessels which run along the bottom of the ambulacral furrows of the arms. 



4. The muscular System. 



We have already several times had occasion to mention this system. In the Bri- 

 singa there are a very great number of muscles, but they are mostly very small, and of an 

 extremely simple structure. Not only are all the single vertebrae or joints in the ambulacral 

 skeleton of the arms movably connected together by short muscles going from one to the 

 other in the periphery of the articulating surfaces of the single calcareous plates, but even 

 every one of the spines situated along the sides of the arms and oh the underside of the 

 disc, is provided with such short muscles which move the spine in different directions ; nay 

 every single one of the microscopic pedicellarise, which in immense numbers cover the cu- 

 ticular sheaths of all the spines and the soft transversal ridges on the dorsal side of the 

 arms, has, as already noticed, its special muscular apparatus. All these muscles seem 

 however, as before stated, to be of extremely simple structure, and should properly be re- 

 garded merely as contractile ligaments, the smooth fibres of which are all parallel. But the 

 muscular system seems to be more developed in the water-feet, and especially in the 

 extremely contractile bucal membrane. Here we find several distinct layers of muscles, 

 the fibres of which cross each other in different directions, and thereby bring about a more 

 complete mobility of the said parts. 



