THE LAEVA OF ASTERIAS VULGARIS. 119 



lated. Even the nnmerous attempts to unite them with the 

 other classes through the agency of larval forms have been 

 more or less unsatisfactory. 



We are asked to believe that in the life-history a form has 

 been secondarily interpolated for the purpose of securing a 

 wider distribution through a prolongation of the free-swim- 

 ming condition. There is no doubt but that many characters 

 of the larva are coenogenetic modifications^ but these are of 

 little importance when compared with those which appear to 

 be ancestral. (1) The cleavage^ total and very nearly equal, 

 and the ciliated cceloblastula, offer simple ancestral condi- 

 tions, and furnish means at the same time for wide distribu- 

 tion. (2) The mode of mesenchyme formation found in the 

 Echinoderms is probably more primitive than the formation 

 of the third germinal layer in the form of mesoblastic bands; 

 and the derivation of this middle layer from any part what- 

 ever of the entoderm is antecedent to that condition where it 

 is restricted to two special cells, the mesoblasts. The nature 

 of the mesenchyme, too, filling up the cleavage cavity with 

 its network of branching cells, is evidence of its primitive 

 character. (3) The formation of enterocoels by archenteric 

 diverticula is characteristic of ancestral forms. And in this 

 larva we find this simplest condition of complete separation of 

 enterocoels and archenteron, passing directly into correspond- 

 ing parts of the adult. (4) The enterocoels open to the 

 exterior very early by definite pores. The presence of two 

 bilaterally symmetrical water-pores at a certain stage in the 

 ontogeny, and the subsequent disappearance of the right pore, 

 seems to point distinctly to the ancestral significance of the 

 larval form ; for we are not justified in supposing that such a 

 character is newly acquired, but that it is ancestral, as Pro- 

 fessor Brooks has pointed out (4). It can only be in course 

 of elimination from the ontogeny. The cause of this disap- 

 pearance of the right pore may be traced to the subsequent 

 connection between the two enterocoels by fusion in the pre- 

 oral lobe. (5) The formation of the pore canals from ecto- 

 dermal and mesodermal elements is similar to the condition 



