12 



decreases from the base to tlie tip (PL I., fig. 0; PL II., 

 fig. 24; PL III., figs. 32 and 38). The penultimate and 

 terminal segments are armed with minute hooks (PL III., 

 hg. 34). The axial canal which traverses the arm branches 

 into all the pinnules and ends blindly in the terminal 

 segment of each. The movements of the pinnules are 

 effected by flexor muscles lodged in deep notches in the 

 ventral margin of each articular face, and extensor fibres 

 which pass from segment to segment on the dorsal margin. 

 Ligamentous fibres only bind the proximal segment to 

 the brachial segment that bears it. The pinnules borne 

 by the second brachial segments are usually twice the 

 length of the succeeding ones, and are distinguished as 

 the oral pinnules (PL L, fig. 3; PL II., fig. 24; PL III., 

 fig. 32). They have no tentacles and no ambulacral 

 grooves, and during life are found more or less strongly 

 flexed over the tegmen calycis (fig. 24). The next 

 pair are quite short, but the succeeding pairs gradually 

 increase in length until the middle portion of the arm 

 is reached, beyond which the length decreases to its distal 

 end or growing point. Uichotomous division occurs 

 repeatedly at the latter, and one branch, on the right and 

 left sides alternately, remains short and constitutes a 

 pinnule. . 



The external surface of the disc and arms is every- 

 where invested by a delicate cuticle, beneath which the 

 cells of the ectoderm are distinguishable only in the 

 ambulacral grooves, the inner faces of the oral and 

 marginal tentacles, and upon the pinnular tentacles, where 

 they form sensory 2)a2)ill8e. 



In other parts, notably along the sides of the arms 

 and the lateral and dorsal surfaces of the visceral mass, 

 the most superficial cells beneath the cuticle have a more 

 or less regular arrangement, but no sharj) distinction can 



