50 EMBRYOLOGY OF THE STAJRFISH. Part I. 



placing lowest, in the scale, all those Starfishes which have, like Ctenodiscus, a pen- 

 tagonal outline, and at the same time pointed tentacles; next in order would come 

 the Starfishes with pointed rays and pointed tentacles, without suckers, like Luidia 

 and Astropecten; above them pentagonal Starfishes, with plates like Anthenea and 

 Hippasteria, and two rows of tentacles, provided with suckers ; then those with more 

 prominent rays, and tentacles also ending in suckers, like Pentaceros and Artocreas ; 

 higher still, the Starfishes, with long slender arms, and only two rows of tentacles 

 with suckers, such as Cribrella, Ophidiaster, and the like ; while highest in the 

 order we should place the genuine Asteracanthion, with four rows of tentacles, with 

 suckers, and highly-developed spines on the abactinal area. 



The same principles applied to the different families would place Starfishes, having 

 plates without spines, lower than those in which the net-work of limestone is cov- 

 ered with spines on the abactinal surface. This classification is not very different, 

 as far as regards the order of the three families proposed by Miiller and Troschel. 

 It differs materially, however, in the standing ' given to pentagonal Starfishes in a 

 short paper by Professor Agassiz, in the Proceedings of the Natural History Society 

 of Boston. From this it is plain, that the mere study of the adult is not a sound 

 foundation for a natural classification. The echinoid characters of the young Starfishes 

 were not known at that time, which would naturally give the pentagonal Starfishes 

 an entirely different position. Nor is it always sufficient to have traced the devel- 

 opment 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. Etnbryologists should, 

 therefore, 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 maclreporic body on the same area, are features of 

 the Crinoids and Ophiurans. These peculiarities are soon lost, and the maclreporic 

 body gradually finds its way upon 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. If we were to imagine a stem on 

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

 abactinal side, would have all the appearance of a Crinoicl. 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 



