440 COMPARATIVE AX ATOM Y CHAP. 



B. The Braehial Cavities. 



1. Asteroidea. The body cavity of the disc is produced in the 

 form of large cavities which run along the arms to their tips. The 

 body cavity is thus found throughout the whole of that part of the 

 arms which is enclosed by the skeletal plates, and contains (1) the 

 ampullae of the water vascular system, (2) the two radial caeca of the 

 stomach, and finally (3) a part of the genital glands as well. The 

 radial caeca of the stomach (two of which occur in each arm) are each 

 attached to the dorsal wall of the arm by two suspensors, which run 

 in the longitudinal direction, so that, above each caecum there lies a 

 coelomic canal whose walls are formed (1) dorsally by the brachial 

 wall, (2) ventrally by the wall of the caecum, and (3) laterally by the 

 two suspensors (</. Fig. 354, p. 411). 



2. Ophiuroidea (Fig. 355, p. 412). The vertebral ossicles occupy 

 so large a part of the transverse section of the arm, that only a very 

 small space is left for the brachial cu'lom. This latter is found as a 

 flat cavity below the dorsal wall of the arm. It is divided into con- 

 secutive chambers, which agree in number with the segments of the 

 arm ; these chambers are incompletely separated from one another by 

 transverse, vertical, calcareous septa, which connect the vertebral 

 ossicles with the outer skeletal plates of the arm. These septa leav. 

 a mediodorsal space free, through which all the chambers are in open 

 communication with one another. This is the " dorsal canal " of 

 authors. 



The endothelium of the brachial cavity is thickened in the dorsal middle line, 

 and carries specially strong cilia. There is thus in each arm a longitudinal ciliated 

 band or streak, which, when it reaches the disc, passes into the ordinary endo- 

 thelium. The activity of these strong cilia originates and maintains the circulation 

 of the body fluid in the brachial cavity. Occasionally the ciliated streak is deepened 

 into a groove. 



3. Crinoidea (Fig. 356, p. 413). In this class also the brachial 

 cavity is much reduced by the strong development of the skeletal 

 joints of the arms ; unlike that of the Ophiuroidea, however, it is 

 found on the ventral side of the arm. The cavity is divided by a 

 horizontal longitudinal septum into two canals, one lying above the 

 other ; both of these run through the arm and its branches as far as 

 to the tips of the pinnulae. The dorsal (apical) coelomic canal is 

 known as the dorsal, and the ventral (oral) canal as the ventral or 

 subtentaeular canal. The latter, on reaching the disc, enters the 

 axial coelom, the former enters the peri-intestinal cavity. The ventral 

 canal itself, again, is divided by a vertical longitudinal septum into two 

 lateral canals. 



The endothelium of the dorsal canal occasionally (especially in the pinnula 1 ) 

 shows small sac-like bulgings, which are known as ciliated baskets or sacs. The 



