ECHINODERMA TA 



237 



branch of the fork alternately, right and left, remains small 

 and constitutes a pinnule, a method of branching which occurs 

 in plants, and iS termed 

 by botanists scorpioid 

 dichotomy. 



The arms and their 

 pinnules have a grooved 

 ciliated ventral surface, 

 at the disk the grooves of 

 the two arms of a pair 

 unite, and the five grooves 

 thus formed run to the 

 mouth. The arms are 

 flexible, and the free Crin- 

 oids swim through the sea 

 by the graceful undula- 

 tions of these processes. 



In a transverse section 

 of the arm the following 

 parts may be distin- 

 guished : dorsally a large 

 brachial ossicle which is 

 traversed by an axial 

 nerve, the contiguous 

 Ossicles being united and 

 moved by a pair of muscles 

 (Fig. 140). Ventral to 

 the ossicle is the body- 

 cavity broken up into 

 four spaces which com- 

 municate atintervals. One 

 of these is dorsal, one ven- 

 tral, and two lateral, the ventral portion is traversed by the sterile 

 generative rhachis. Below these coelomic spaces lies the radial 

 water-vascular vessel which gives off alternating branches to 

 the nonsuctorial tube -feet. At the side of the ambulacral 

 groove some spherical bodies of unknown function are situated, 

 these are termed sacculi, and consist of a membrane enclosing a 

 large group of spherules. 



Fig. 139. — Pentacrinus caput Medusae. 

 After Gxittard. 



