and Enibryological Development. 351 



seas. Ample material for making this comparison is fortu- 

 nately at hand ; it is material of a peculiar kind, not easily 

 obtained, and which thus far has not greatly attracted the 

 attention of zoologists. 



Interesting and important as are the earliest stages of 

 embryonic development in the different classes of the animal 

 kingdom, as bearing upon the history of the first appearance 

 of any organ and its subsequent modifications, they throw 

 but little light on the subject before us. What we need for 

 our comparisons are the various stages of growth through 

 which the young Sea-urchins of different families pass, from 

 the time they have practically become Sea-urchins until they 

 have attained the stage which we now dignify with the name 

 of species. Few embryologists have carried their investiga- 

 tions into the more extended field of the changes the embryo 

 undergoes when it begins to be recognized as belonging to 

 a special class, and when the knowledge of the specialist is 

 absolutely needed to trace the bearing of the changes under- 

 gone and to understand their full meaning. Fortunately the 

 growth of the young Echini has been traced in a sufficient 

 number of families to enable me to draw the parallelism 

 between these various stages of growth and the palaeon- 

 tological stages in a very different manner from what is 

 possible in other groups of the animal kingdom, where we 

 are overwhelmed with the number of species, as in the 

 Insects or Mollusks, or where the palasontological or the 

 embryological terms of comparison are wanting or very 

 imperfect. 



Beginning with the palasontological history of the regular 

 Sea-urchins of the time of the Trias, when they constituted 

 an unimportant group as compared with the Crinoids, we 

 find the Echini of that time limited to representatives of 

 two families. One of these, the genus Cidan's, has con- 

 tinued to exist, with slight modifications, up to the present 

 time, and not less than one tenth of all the known species 

 of fossil Echini belong to this important genus, which in 

 our tropical seas is still a prominent one. It is interesting 

 here to note that in the Cidaridse the modifications of the 

 test are not striking, and the fossil genera appearing in the 

 successive formations are distinguished by characters which 

 often leave us in doubt as to the genus to which many 

 species should be referred. In the genus Rhabdocida?ns, 

 which appears in the lower Jura, and which is mainly cha- 

 racterized by the extraordinary development of the radioles, 

 we find the extreme of the variations of the spines in this 

 family. From that time to the present day the most striking 



