Chap. III. EMBRYOLOGI CAL CLASSIFICATION. 49 



tracing the more complete affinities among animals. One of the principal reasons 

 why embryologists have overlooked these investigations, may be found in the fact 

 that they rarely examine more than one species of each type at a time. Who 

 would place the young Echinus, with its Cidaris-like spines and straight simple ambu- 

 lacra, among the true Echinida?, or take a young Spatangoid for anything but an 

 Echinus? "What has the pear-shaped outline and long tentacles of a young Bolina 

 — which is, indeed, a diminutive picture of a Pleurobrachia — in common with the 

 adult, with its long, twisting rows of ambulacra, and wing-like projections of the 

 spheromeres beyond the actinostome ? Yet these embryonic characters remind us of 

 familiar forms, and cannot fail to give us an insight into the relative standing of the 

 forms through which they pass. 



Let us commence with our embryo Starfish at the time when it is just forming, 

 and when the first outlines of the abactinal region can be traced. Suppose its 

 development were to stop there (PI. V. Fig. 5), that the slight lobes should close, 

 soon after the formation of the coating of limestone granules over the abactinal 

 area, we should have a condition strongly reminding us of a Culcita, with its arched 

 back, its almost circular outline, and the total absence of any very prominent spines. 

 In the next stage (PI. V. Fig. 12), the cuts between the rays have become somewhat 

 more marked, the plates of limestone cells are well developed, and there are tuber- 

 cles in place of future spines. The resemblance of this stage to such forms as 

 Anthenea, Pentagonaster, and the pentagonal Starfishes, in which we find a great 

 development in the abactinal plates, is at once apparent. In a somewhat more 

 advanced stage, the rays are slightly more marked, the spines quite well developed ; 

 this type is represented among living Starfishes by such forms as Pteraster, Paulia, 

 Pentaceros, Artocreas, Oreaster: unless it were known beforehand that PL VIII. Fig. 1 

 represents a highly - magnified young Starfish, the figure would readily pass for 

 a new species of Oreaster. The corresponding changes of the actinal surface are 

 not the less important. In the early stages, the tentacles are pointed, they have 

 no disk (PI. VI. Figs. 3, 9) ; it is only afterwards that they are developed (PI. VII. 

 Fig. 1 ; and PL VI. Figs. 10, 11, 12). In fact, the tentacles of our young Starfish, 

 in its earlier stages, resemble those of Astropecten, Luidia, and Ctenodiscus. We 

 are, therefore, at once provided with a set of characters taken from the young, 

 enabling us to decide the comparative value of the various features, and the order 

 in which they are to be taken. From the tentacles alone we are fully justified, 

 upon embryological data, in placing Starfishes with pointed tentacles lower than 

 those which have disks, like Asteracanthion. Another embryological feature is the 

 fact that the embryo has only two rows of tentacles, while in the adult Astera- 

 canthion we find the tentacles arranged in four rows. Combining these characters, 

 as we find them in the adults, we have at once good and conclusive reasons for 



VOL. v. 7 



