508 



INVERTEBKATA 



CHAP. 



anterior .end of the larva. No vacuolated crests of any kind are 

 developed, but with this exception the subsequent development, up 

 .to the age of six -days, is, point for point, identical with that of 

 Ophiothrip. The separation of' the coelom from the archenteron, 

 its division into two sacs, the formation of the madreporic pore, 

 the formation of the stomodaeum, the differentiation of the gut, 

 the formation of the adoral band of cilia, and of the longitudinal 

 locomotor band of cilia, of the first four larval arms, and of their 

 supporting calcareous rods, take place in precisely the same way as 

 in the Ophiopluteus larva and need not be specially detailed here. 

 No wonder that the Echinoid larva, when discovered by Johannes 



coe 



arch 



cole 



p.mes 



FIG. 384. Early stages in the development of Echinus esculentus. (Original.) 



A, blistula. B, advanced gastrula, showing the formation of the coelom. arch, nrchenteron ; Wp, 

 blastopore ; calc, calcareous star ; cil, apical tuft of specially long cilia ; coe, coelomic vesicle arising as 

 an outgrowth from the archenteron ; p.mes, primary mesenchyme cells. 



Miiller, was regarded by him as another species of his genus " Pluteus." 

 It is now termed Echinopluteus in order to distinguish it from the 

 Ophiuroid larva. The sole differences between the two which can 

 be detected at this stage concern (1) The posterior ciliated arms ; 

 these are more closely approximated to one another than in the 

 Ophiopluteus, and, as subsequent development shows, correspond to 

 the post-oral arms of that larva, not to those which are just formed 

 in the Ophiopluteus, i.e. the postero - lateral. The Echinopluteus 

 in this stage is consequently rather more square in section not so 

 flattened, in fact, as the Ophiopluteus. (2) The body-rods; these, 

 instead of ending in the aboral pole in bifurcations, as is the case in 

 the Ophiopluteus larva, end in inbent crooks, which may branch 

 slightly but which f do not unite with their fellows on the opposite 

 side of the body. 



