520 INVERTEBRATA CHAP. 



which one or more stones covered with the calcareous tubes of Serpulids 

 are placed, it seems to find food and conditions congenial to it, and it 

 rapidly grows in size. The paired tube feet enlarge and become 

 functional ; the oral surface grows more quickly than the aboral one, 

 and the primary tube feet are thus forced upward. As new pairs of 

 tube feet are developed in each radius between the first-formed pair 

 and the primary azygous one, the latter becomes of less importance, 

 ceases to grow, and becomes enclosed in grooves in the terminal 

 plates, which have greatly increased in size. 



Alternating with the five terminals are five plates, the so-called 

 basals, one of which surrounds the water-pore, and which had already 

 appeared in the late larva. On the ventral surface, in the substance of 

 the epineural veil, ten plates appear, two in each interradius. Beneath 

 each pair the tip of the incipient tooth can be seen (von Ubich, 

 1913). The teeth probably correspond to the mouth angle plates of 

 Ophiuroids and Asteroids ; the paired plates, which are the rudiments 

 of the alveoli or jaws, to the first pair of adambulacral plates in 

 the arms of Asteroids and Ophiuroids (j, Fig. 391, A). These corre- 

 spond to the so-called ambulacral plates of the test of Echinus, which 

 are external to the nerve cord, not internal to it as are the true 

 ambulacrals of Asteroidea. The internal position of the first pair 

 of " adambulacrals," which form the jaws, is correlated with the in- 

 vagination of the ectoderm round the mouth, in Echinoidea, to form 

 an adult stomodaeum. 



When once the epineural veil is torn and the mouth becomes 

 functional, the teeth grow quickly and project externally as long 

 pointed spines, presenting quite a different appearance to their 

 retired situation when adult. Numerous spines are added, but these 

 are all similar to those which were formed from the floor of the 

 amniotic cavity. The quadrangular immovable spines are confined to 

 the aboral surface, which is rapidly diminishing in relative size. 

 New pedicellariae are formed, and especially conspicuous are five 

 "sphaeridia," one in each radius. These sphaeridia are short-spherical 

 spines in which the calcareous skeleton is a clear solid mass. 



When the little urchin has doubled its diameter the anus is 

 formed. As the larval intestine has persisted as a blind pouch, 

 lying in the mesentery, separating left from right posterior coeloin 

 (now oral from aboral), and as the new anus is formed by a growth 

 of the intestine along this mesentery, it is obvious that the new anus 

 is formed about where the old one became closed. 



The larval stomach is transformed into the first or lower coil of 

 the adult gut ; the second or recurrent coil is gradually formed by 

 the increase in length of the larval intestine ; since the mouth and 

 anus are fixed points, this increase in length can only be relieved 

 by the bending back of the gut into the second coil (Fig. 392). 



The genital rachis and the protecting aboral sinus are formed 

 just as in Asteroidea and Ophiuroidea. From the rachis five 

 interradial strands grow out which form the beginnings of the genital 



