326 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



surround a modern terrestrial larva must, in nearly every case, be very different from those under 

 which the remote ancestors of the species passed their life, but while this is also true, to some 

 degree, of marine animals their inorganic environment is comparatively stable, and the persistence 

 of so many ancient marine types shows that the changes in the organic surroundings of marine 

 animals take place much more slowly than corresponding changes on land. 



This fact, joined to the definite character~bf the changes which make up the life history of a 

 marine crustacean, renders these animals of exceptional value for the study of the laws of larval 

 development, and for the analysis of the effect of secondary adaptations, as distinguished from the 

 influence of ancestry ; for while Clans has clearly proved that adaptive larval forms are much 

 more common among the Decapods than had been supposed, his writings and those of Fritz Muller 

 show that no other group of the animal kingdom presents an equal diversity of orders, families, 

 genera, and species in which the relation between ontogeny and pliylogeny is so well displayed, 

 but, while proving this so clearly, Claus' well known monograph also shows with equal clearness 

 that this ancestral history is by no means unmodified, and that the true significance of the 

 larval history of the higher Crustacea can be understood only after careful and minute and exhaus- 

 tive comparison and analysis. 



Greatly impressed by this fact, I began nearly ten years ago to improve the opportunities 

 that were offered by the marine laboratory of the Johns Hopkins University, for obtaining more 

 complete and detailed knowledge of the larval stages of a number of Macroura, and this work has 

 been prosecuted at every opportunity up to the present time. Some of my results have been pub- 

 lished in my monograph on Lucifer, in the Phil. Trans. Royal Soc. for 1882, and others are incor- 

 porated in my report on the Stomatopoda collected by H. M. S. Challenger. 



This memoir contains the life histories of a number of additional species based in part 

 upon my own studies at Beaufort, North Carolina, and at Green Turtle Key and New Providence 

 in the Bahama Islands, but chiefly upon the researches which one of my students, Mr. F. H. Her- 

 rick, has carried on under my general supervision. In 1SSO he undertook, at my suggestion, the 

 study of the embryology and metamorphosis of the Macroura, and devoted three years to this sub- 

 ject under my direction, and the results which follow are almost entirely due to his zeal and 

 energy. He has completed the study of several subjects upon which I had previously made a 

 beginning, so that my own unfinished notes have been incorporated with his researches, and our 

 respective shares in the work are as follows: The chapter on Gonodactylus is entirely based upon 

 my own researches; the chapter entitled " Alpheus, a study in the development of the Crustacea," 

 is entirely the work of Mr. Herrick ; the one on the metamorphosis of Alphens is based upon our 

 combined studies, and that upon Steuopus is almost entirely the work of Mr. Herrick, as my own 

 contributions to this life history are of minor value except so far as they supplement his work. 



I shall now give a brief outline or summary of the chief results which are described in detail 

 in each chapter. 



THE LIFE HISTORY OF STENOPUS HISPIDTIS. 



During the six seasons which I spent at Beaufort, North Carolina, I captured in the tow-net, 

 at different times, some six or seven specimens of a remarkable pelagic crustacean larva, all of 

 them well-advanced and in nearly the same stage of development. 



Nothing was learned of the earlier larval life nor of the adult form of the animal, although 

 enough was made out to show that it is one of the few Macroura which, like Peneus and the Ser- 

 gestidai, have retained tbe primitive or ancestral metamorphosis, and that its secondary modifica- 

 tions are very slight as compared with those of ordinary macrouran larvse, and also that the 

 Beaufort larva? are new to science. (See Pis. is and x.) 



These larvse have the full number of adult somites and appendages, and in side view they are 

 very suggestive of the Sergestid;e. They are very much larger than ordinary pelagic larvae and 

 are quite different from any known forms of Macronra. 



The chief locomotor organs are the last pair of thoracic legs, which are extremely slender, as 

 long as the entire body of the larva, ending in flattened elliptical paddles, which are used as 

 sweeps for rowing through the water. They are stretched out in front of the body near the 



