XVI ECHINODERMATA 515 



cavity. It is filled with mesenchyme cells which have been budded 

 from the wall of the left posterior coelom. These cells arrange them- 

 selves in a network, in the interstices of which appears calcareous 

 matter which consequently assumes the form of a lattice-work — a 

 negative, so to speak, of the framework of cells to which it owes its 

 origin. The bases of these spines are thickened so as to look like 

 collars, and here the contained mesenchyme undergoes transformation 

 into the muscles which attach the spine to its boss. The overlying 

 ectoderm develops nervous fibrils. 



The perihaemal pockets begin to send off narrow outgrowths 

 between ectoderm and outgrowth of hydrocoele, and form the two 

 radial perihaemal canals in each radius, which, in the adult, fuse 

 into one (p.h.r, Fig. 388). The body of each perihaemal space remains 

 larger than it does in the Asteroids and Ophiuroids, and on its outer 

 wall a protrusion appears which projects into its cavity. This pro- 

 trusion is the formative tissue for one of the teeth, and the body of 

 the perihaemal pocket into which it projects forms one of the five 

 dental sacs which, collectively, form the cavities of Aristotle's 

 lantern. 



It is characteristic of the later stages of metamorphosis that the 

 cavity of the hydrocoele ring becomes enormously distended with 

 fluid. In the centre of this ring there appears an invagination of 

 the ectoderm which constitutes the lower floor of the amnion. This 

 invagination is the adult stomodaeum, a structure which does not 

 exist in Asteroidea and Ophiuroidea; a small peg-like outgrowth 

 protrudes from the stomach towards it, which eventually meets the 

 adult stomodaeum and the two fuse, and so the adult mouth is formed. 



If this description has been followed it will be seen that the 

 adult mouth is at first shut off from the exterior, not merely by the 

 roof of the amniotic cavity but also by the epineural veil. The 

 swelling of the Echinus rudiment has indented the larval stomach ; 

 it is no longer globular, but hemispherical, a flattened side being 

 turned towards the Echinus rudiment. 



Whilst these changes have been occurring and the Echinus 

 rudiment has been growing in size, other changes have been occurring 

 in other parts of the larva. From the tips of the re-entrant angles 

 of the ciliated band, which are situated between the postero-lateral 

 and post-oral arms, a pair of ciliated epaulettes become separated 

 off (p.cil.ep, Figs. 387 and 390). This extra pair may be termed the 

 posterior ciliated epaulettes. They are characteristic of the larva of 

 Echinus esculentus ; they do not appear in the larva of E. miliaris. 

 They make their appearance about the twenty-fifth day. 



At the same time three pedicellariae of the ophicephalous type 

 make their appearance on the larva ; each arises as a little knob-like 

 outgrowth of the ectoderm, into which mesenchyme cells are budded 

 from an adjacent area of the coelom. Some of these cells form the 

 skeleton of the organ and some the muscles. One of these pedicellariae 

 is situated near the aboral pole of the larva; this one is not 



