60 EMBRYOLOGY OF THE STAEFISH. 



and we necessarily lose many elements of the greatest importance, when- 

 ever we attempt to associate the adults of an}^ class in natural groups, 

 without taking into account the characters of their young. Natural- 

 ists, who have not yet entered upon this method of study, cannot con- 

 ceive what extraordinary facilities this kind of investigation affords for 

 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, w^ith its Cidaris- 

 like spines and straight simple ambulacra, among the true Echinidse, 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, in- 

 deed, 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 char- 

 acters 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 al)actinal region can be traced. 

 Suppose its development were to stop there (PI. Y. Fig. 5), and that 

 the slight lobes should close soon after the formation of the coating of 

 limestone granules over the abactinal area, we should then have a condition 

 strongly reminding us of a Culcita, with its arched back, its almost circu- 

 lar outline, and the total absence of any very prominent spines. In the 

 next stage (PI. Y. Fuj. 12), the cuts between the rays have become some- 

 what more marked, the plates of limestone cells are well developed, and 

 there are tubercles 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 rep- 

 resented among living Starfishes by such forms as Pteraster, Paulia, Pen- 

 taccros, Artocreas, Orcaster: unless it were known beforehand that PI. 

 A'lll. Flff. 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, 0); 



