21 



Prof. Miiller calls the stomach, and which doubtless sur- 

 rounds the cavity to which the mouth and pharynx conduct. 



The first evidence of the change to the Echinus is the 

 formation of a circular disc, like a clock-face, on one side 

 of the granular mass : five double lines, like pointers, radiate 

 from the centre of this disc, and their extremities expand 

 into circles with a double outline. These circles form the 

 bases of as many hollow tentacles, like those that traverse 

 the ambulacral pores of the adult Echinus, but which are 

 here single, not in pairs. Triradiate pedicellariae appear, 

 two on each side the disc, of the kind called ' gemmiform ' 

 in the adult Echinus. The disc progressively expands, ex- 

 tending over the cellular mass to which it owes its origin, 

 anddevelopes tubercles, which push through the transparent 

 dome of the Pluteus, and are transformed into spines and 

 tentacles, both of which, together with the pedicellariae, 

 manifest their characteristic motions and combine these 

 seemingly voluntary actions with the automatic continued 

 vibrations of the ciliated epaulets of the larva ; so that it 

 can now both swim and creep. By the time the disc has 

 grown over half the granular sphere little of the larva re- 

 mains, except some of the slender calcareous rods : the per- 

 forated oviducal plates of the nascent Echinus are formed 

 round the pentagonal space, as yet imperforate, in the 

 centre of the original disc, determining the anal pole of the 

 developing spiny globe, close to which the cicatrix of the 

 ruptured pharynx of the Pluteus is completed by the so- 

 called ' madreporiform plate/ The mouth of the Echinus 

 is quite distinct and remote from that of the Pluteus, and 

 is first indicated in the centre of the naked or unspined 

 moiety of the granular sphere by the formation of the hard 

 summits of the five characteristic jaws and teeth of the 

 Echinus. The outer skeleton is completed by the progres- 

 sive extension of the ' ambulacral' and ( interambulacral/ or 

 spine-bearing plates, and by the less regular 'oral' plates. 



Thus the Echinus is fully manifested, and becomes either 



