380 



COMPA EA TI VE A NA TOM Y 



(r) Two rows of side plates (o). 



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



{(') Two rows of pinimlfP (2). 



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



(g) A double row of covering plates ( 1 ). 



Fig. 331.— Diagram of tlie organisation of a Pentremites (original). A, B, C, D. E, tlie five 

 ambulacra. A, Aiiibiilacruiu with covering plates (1) and extended pinnula- (-J). B, Anibulacnini 

 with depre.ssed pinnula-. C, Ambulacrum after remo\al of the pinnute and covering plates. D, 

 After the further removal of the side plates and accessory side plates (exceiit 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, pinnulse ; 3, deltoid plates; 4, their 

 sloping ambulacral edges ; 5, side plates ; (5, lancet plate ; ", pores ; S, 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 tlie covering plates, wliieli are very rarely retained, we have, at the 

 centre of each ambulacrum, a skeletal plate about half as wide as the ;uul>ulacriim 

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



