330 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



of larval stages, the life history of those which are found at Key West is very different from that 

 of those which live on the coast of North Carolina, while those which we studied in the Bahama 

 Islands present still another life history. In the case of the second species Alpheus saulcyi the 

 difference stands in direct relation to the conditions of life. The individuals of this species inhabit 

 the tubes and chambers of two species of sponges which are often found growing on the same 

 reef, and the metamorphosis of those which live in one of these sponges is sometimes different 

 from thai of those which inhabit the other. In this species the adults also are different from 

 each other, but as we found a perfect series of transitional forms there is no good reason for 

 regarding them as specifically distinct, and in the case of the other species Alpheus heterochelis 

 we were unable, after the most thorough and minute comparison, to find any difference whatever 

 between adults from North Carolina and those from the Bahama Islands, although their life histories 

 exhibit a most surprising lack of agreement. In fact, the early stages in the life of Alpheus hete- 

 rochelis in the Bahama Islands differ much less from those of Alpheus minor or Alpheus normani 

 than they do from those of the North Carolina Alpheus heterochelis, and, according to Packard, the 

 Key West heterochelis presents still another life history. 



In the summer of 1881 1 received the American Naturalist with Packard's very brief abstract 

 of his observations at Key West upon the developmeutof Al/>hens heterochelis, and read with great 

 surprise his statement that this species has no metamorphosis, since, while still inside the egg, it 

 has all the essential characteristics of the adult. As I had under niy microscope at Beaufort on 

 the very day when I read his account a newly hatched larva of the same species and was engaged 

 in making drawings to illustrate the metamorphosis of which he denies the existence, and as my 

 experience in the study of other Crustacea had taught me that all the larvje of a species at the 

 same age are apparently facsimiles of each other down to the smallest hair, Packard's account 

 seemed absolutely incredible, and I hastily decided that, inasmuch as it was without illustra- 

 tions and was written from notes made many years before, it involved some serious error and was 

 unworthy of acceptance. This hasty verdict I now believe to have been unjust, since my wider 

 acquaintance with the genus has brought to my notice other instances of equally great diversity 

 between the larva? of different specimens of a single species. 



The phenomenon is, however, a highly remarkable one and worthy the most thorough exami- 

 nation, for it is a most surprising departure from one of the established laws of embryology the 

 law that the embryonic and larval stages of animals best exhibit their fundamental affinities and 

 general resemblances, while their specific characteristics and individual peculiarities make their 

 appearance later. 



As in most animals the adult life is most important, the adults have a more diversified envi- 

 ronment than the young, and the divergent modification which is continually taking place to 

 perfect the adjustment between such organism and its conditions of life chiefly affects the adults, 

 so that specific characters and the slight differences between varieties or races are usually con- 

 fined to the adults, while the embryos and larva? are, as a rule, more generalized. 



This is true to a marked degree of those animals whose young are nursed or protected or cared 

 for in any way by their parents, and while it is less true of those animals whose independent life 

 begins very early, yet the same law holds with them also ; and the chief scientific value of embry- 

 ology lies in the fact that a knowledge of the early stages in the life of animals enables us to trace 

 their broad affinities and to distinguish them from more recently acquired differences; for the 

 early stages of two related forms of life share in common their more fundamental characteristics 

 and are essentially alike, while the adults differ from each other and exhibit the divergent speciali- 

 zations which are of more recent acquisition. 



It sometimes happens, however, that the early stages of two closely related species differ 

 greatly. This may occur when the larva? of the one species lead a free, independent, life, while the 

 young of the other species .are protected in someway by the parent. For example, the compli- 

 cated metamorphosis which is so characteristic of starfishes is almost totally absent in those star- 

 fishes which are provided with brood-pouches. The same relation may also bo exhibited when the 

 larviB of one species of a genus have become adapted to a mode of life very different from that of 

 the larva? of the other species of the genus. Thus those species of ^Egiuidie whose larva? are para- 



