ECHINODERMA. BLASTOIDEA 



151 



larger than the basals, and form the main part of the 

 calyx. At the upper end of each there is a deep incision, 

 which serves for the reception of the food-carrying area (a); 

 this is usually spoken of as an " ambulacrum," but there is 

 no evidence of the existence of a radial water vessel, and it 

 is doubtful whether this area is really homologous with the 

 ambulacrum of an Echinoid. Above the radials and alter- 

 nating with them occur five smaller plates — the deltoids (d) 

 or inter-radials. The mouth is placed at the summit 

 of the calyx, in the centre, and around it are five other 



B 



-- s 



r 



Fig. 62. Pentremites godoni, Carboniferous. A, side ; B, upper surface ; 

 C, under surface. a, ambulacra ; b, basal plates ; r, radials ; 

 d, deltoids ; s, spiracles around the mouth ; an, anus, x 2. 



openings termed spiracles (s), one of which is larger 

 than the others and includes the anus (an). From the 

 mouth the five ambulacra (a) radiate towards the 

 aboral surface, and are bordered partly by the deltoids 

 but mainly by the radials. Each ambulacrum (fig. 63) 

 consists of the following plates : in the middle is a long 

 pointed plate (I), the lancet-plate, which is traversed by 

 a longitudinal canal, in which a nerve may have been 

 present. On each side of the lancet-plate is a row of 



