62 EMBRYOLOGY OF THE STAEFISH. 



rally give the pentagonal Starfishes an entirely different position. Nor 

 is it always sufficient to have traced the development of any one species ; 

 unless it happen to stand highest in its group, its different phases would 

 not tell us anything of the relative standing of the other members of the 

 group with which the adult is associated. Embryologists should, there- 

 fore, whenever it is possible, select those species for investigation which, 

 upon anatomical evidence, stand highest in their group. 



There are other embryonic features, recalling not simply families of the 

 same suborder, but characters of other lower orders. The situation of 

 the anus on the actinal side, the presence of the madreporic body on 

 the same area, are features of the Crinoids and Ophiurans. These pecul- 

 iarities are soon lost, and the madreporic body gradually finds its way 

 to the abactinal area. The opening of the anus next to the mouth is 

 eminently crinoidal, and it is accompanied by other structural details 

 reminding us still more of that order. Were there a stem on the 

 central plate of its abactinal area, the young Starfish, when seen from 

 the abactinal side, would have all the appearance of a Crinoid. The 

 central plate corresponds to the basal plate (PL VI. Fig. 10), the set of 

 five plates in the angles of the arms to the interradial plates, and the 

 arm-plates themselves to the radial plates of a Crinoid ; and, to make 

 the resemblance still stronger, the anus opens near the mouth, on the 

 same side with it, as in Comatula. This analogy had already been 

 pointed out by Professor Agassiz, in his Lectures on Embryology ; and 

 it shows conclusively that Starfishes are built upon the same plan with 

 other Echinoderms, contrary to the view\s long entertained by Johannes 

 Miillcr. This comparison to the plates of a Comatula can be carried out 

 to its fullest extent, and is exceedingly instructive if made with the 

 young Comatula, of which an admirable figure has been given by Pro- 

 fessor Allman, in his valuable memoir on the prebrachial stage of Coma- 

 tula, in the Memoirs of the Roj^al Society of Edinburgh for 1863. The 

 arrangement strikes one, at once, as identical, though the plates are by 

 no means homologous. The central plate occurs in both, but the most 

 prominent plates, occupying indeed the greater part of the abactinal re- 

 gion of the young Starfish, arc the same plates which eventually develop 

 with others at the base of the arms, those at the anglo of tlio arms 

 being l)ut little developed. It is quite the reverse with Comatula. in 

 which the arm-plates are but small at this stage; though, according to 



