TYPE ECHINODERMA. 555 



lymph, laden with excretory particles, migrate to the exterior 

 through the walls of the branchiae. 



The calcareous plates are imbedded in a connective tissue 

 usually of considerable thickness and sometimes of a high 

 degree of consistency. Upon its inner side are to be found 

 circular and longitudinal bands of muscles, especially devel- 

 oped in the arms, which are capable of considerable move- 

 ment. 



The general ccelom is traversed by mesenteries extending 

 from the body-wall to the digestive tract, which do not require, 

 however, a detailed description. Suffice it to say that each 

 radial caecum of the digestive tract which extends out into the 

 arm is suspended by two longitudinal mesenteries (Fig. 254) 

 which, with the body-w r all and csecurn, enclose a canal open- 

 ing proximally into the general co3lom. Extending vertically 

 through the c?elom in the iuterradius CD is a cavity with 

 strong walls which is in communication with the exterior by 

 the madreporiform tubercle and is the axial sinus. This is a 

 portion of the ccelom which is early separated from the rest, 

 and in the embryo opens to the exterior by the water-pore, 

 the stone-canal opening into it. In the adult the cavity of the 

 sinus is fairly spacious and contains the ovoid gland sus- 

 pended to its walls by a mesentery. Prolongations of the 

 sinus accompany the genital rachis and, enclosing the repro- 

 ductive organs, form the genital sinuses. 



The schizoccelic system consists of an oral ring lying 

 between the nervous and water vascular rings, and of five 

 radial vessels which pass out from this along the axes of the 

 arms. The oral ring is divided by an oblique septum into two 

 portions, one of which lies upon the aboral surface of the 

 other, and enters into connection with the axial sinus, the 

 ovoid gland abutting upon the septum in one of the iuterradii. 

 The oral sinus also communicates with the cceloin by rive 

 interradial orifices. The radial sinuses are divided into two 

 cavities by a median longitudinal septum, and, like the oral 

 sinus, communicate with the crelom. A lacuuar system, such 

 as occurs in the Crinoids, is not developed in the Asteroideu, 

 though certain spaces in the wall of the ovoid gland and its 

 prolongations are perhaps representatives of it. The ovoid 



