380 COMPARATIVE ANATOMY 



(c) Two rows of side plates (">). 



(d) Two rows of accessory side plates (8). 

 (>') Two rows of pinnulre (2). 



(/) Two groups of folds of hydrospire pouches (13). 

 (</) A double row of covering plates (1). 



CHAP. 



-JO 1 



D 



FIG. 331. Diagram of the organisation of a Pentremites (original). A, B, C, D, E, the live 

 ambulacra. A, Aiubiilacruni with covering plates (1) and extended pinnula- (2). B, Ambulacrum 

 with depressed pinmila-. C, Ambulacrum after removal of the pinnula? and covering plates. D, 

 After the further removal of the side plates and accessory side plates (except 3). E, After removal 

 of the lancet plate as well. In the centre is seen the mouth with the spiracles surrounding it, in 

 the posterior interradius the anus. 1, Covering plates; 2, pinnulae ; 3, deltoid plates; 4, their 

 sloping ambulacral edges ; 5, side plates ; 6, lancet plate ; 7, pores ; 8, outer or accessory side 

 pieces ; 9, furrow of unknown significance on each side plate ; 10, radials - fork plates ; 11, aperture 

 of the ambulacral canal ; 12, lower lancet plate ; 13, hydrospire folds. 



Passing over the covering plates, which are very rarely retained, we have, at the 

 centre of each ambulacrum, a skeletal plate about half as wide as the ambulacrum 

 itself, and more or less similar in shape. This is the so-called lancet plate (Fig. 



