XVI ECHINODEEMATA 487 



and the vacuoles almost certainly consist of some material which 

 is lighter than water. Thus, while the general shape of the embryo 

 becomes conical, the outline of its cavity, the blastocoele, remains 

 spherical. 



The blastula now acquires cilia, bursts the egg-membrane, and 

 enters on a free swimming existence. Before eighteen hours have 

 elapsed the invagination to form the archenteron has begun, and 

 the blastula has become a gastrula. Secondary mesenchyme is 

 given off from the apex of the archenteron as this is being formed. 

 "When the archenteron is fully formed it swells out at its free end 

 into a thin-walled vesicle, which is the rudiment of the coelom 

 {coe, Fig. 374, B). In Asterias, as we 'have seen, there is also formed 

 a single vesicle at the apex of the archenteron, but, according to the 

 current account, the coelom originates as two sacs which are cut off 

 separately from the archenteron. 



In Ophiothrix, however, when this vesicle becomes cut off from the 

 archenteron, it persists for a brief time as a single vesicle ; then it 

 becomes divided into right and left halves, which lie at the sides of the 

 gut and form the right and left coelomic sacs. At the same time two 

 lateral outgrowths of the body appear in the larva, which has up till 

 now had a conical shape {p.l.a, Fig. 373 G); these are the rudiments of 

 the posterolateral arms, and into them passes nearly all the primary 

 mesenchyme. The cilia, which until now have covered the whole 

 body, become restricted to a circular band of thickened ectoderm 

 which passes over these arms. This band is, of course, homologous 

 with the longitudinal ciliated band of the Bipinnaria larva. In the 

 mesenchyme at the base of each rudimentary arm there appears a 

 little tri-radiate spicule of calcium carbonate. Of its three rays one 

 extends downwards towards the posterior pole of the larva, this is 

 termed the body-rod; one extends outwards into the arm and is 

 termed the postero-lateral rod ; and the third extends in an anterior 

 direction and is termed the antero-lateral rod {a.l.a, Fig. 375, A). 



The stomodaeum makes its appearance just behind the vacuolated 

 crest on what will prove to be the ventral side of the larva. As it is 

 being formed, the gut, from which the coelom has detached itself, 

 becomes marked out into three regions, viz. oesophagus, stomach, 

 and intestine, by the appearance of two constrictions. The 

 stomodaeum joins the oesophagus, and when this has been accomplished 

 the alimentary canal is complete. The opening of invagination, or 

 blastopore, persists as the larval anus. An adoral band of cilia is 

 formed just in the same way as it is formed in the Bipinnaria, and 

 doubtless fulfils the same function. At the same time the two larval 

 arms grow rapidly in length, and the portions of the ciliated band 

 which pass over them are pulled out into long loops. The vacuolated 

 crest diminishes in size and soon disappears completely, and the whole 

 organism takes on in consequence the form of a V. The growth of 

 the larval arms appears to be due to the growth of the calcareous 

 rods contained in them, not because the growth of the arm is due to a 



