380 



COMPARATIVE ANATOMY 



CHAP. 



((■) Two rows of side plates (5). 



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



(e) Two rows of pinnule (2). 



(/) Two groups of folds of' hydrospire pouches 

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



13) 



Ji. 



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

 ambulacra. A, Aiiibiilacruni with covering plates (1) and extended pinnulw (2). B, Ambulacrnin 

 with depressed pinnulic. C, Ambulacrum after removal of the pinnulie and covering plates. D, 

 After tlie further removal of the side plates and accessory siile 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 po^tpriur interradius tht* anus. 1, Covering plates; 2, pinnulte; 3, deltoid plates; 4, their 

 sloping ambulacral edges ; 5, side plates ; G, 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. 



