A. HYATT ON TETRABRANCHIATE CEPHALOPODS. 193 



VII. On the Parallelism between the different stages of Life in the Individual and those in the 

 entire Group of the Molluscous order Tetrabranchiuta. By Alpheus Hyatt. 



Read February 21, 18C6. 



.HERETOFORE all investigators of the morphological changes taking place in the indi- 

 vidual during its life have directed their attention wholly to the examination of the 

 younger stages, while the body is increasing in bulk, and the various organs are developing 

 from an immature to a more or less matured condition. 



The importance of continuing such researches throughout the entire life of the individ- 

 ual has not yet attracted the attention of naturalists, although, as I hope to show, the mod- 

 ifications which succeed the embryological stages, during the adult period and old ao-e, or 

 period of decline in the individual, are not less significant, or less worthy of the deepest 

 study. 



No one, with the exception of Alcide D'Orbigny, has yet attempted to describe system- 

 atically all the metamorphoses of an individual from its ovarian origin to its death, and 

 even he has not detected the correlations which exist between these metamorphoses and 

 the gradual changes which mark the course of life in the group, from its beginning in time 

 to its extinction. D'Orbigny's observations were made upon the Ammonites, and having 

 been engaged myself, for the past six years, in arranging the magnificent collections of 

 Cephalopods belonging to the Museum of Comparative Zoology at Cambridge, Massachu- 

 setts, I have had ample means of testing the truth of his statements upon European speci- 

 mens. The Museum collections are very richly furnished with Ammonites, nearly every 

 species represented by a large number of individuals, and comprising, among others, the 

 former cabinets of Prof. Bronn, L. de Koninck, M. Boucault, M. Duval, a large and choice 

 selection purchased from Dr. A. Krantz of Bonn, and a suite of exchanges from the Mu- 

 seum of Stuttgardt. These having been placed temporarily in my charge, to be arranged 

 in the show-cases designed for them, I availed myself of this favorable opportunity, while 

 connected with the Museum in the capacity of a student, and since my graduation, to scru- 

 tinize closely the account given by D'Orbigny of the young, adult, and period of decline 

 of the Ammonite. 



These studies 1 finally led me to compare the characteristics of the period of decline 

 the individual with the adult features of allied species in the hope of finding similar corre- 

 lations to those so successfully worked out by Von Baer, Professor Louis Agassiz, and 

 others, between the development of the young and the permanent states of simpler organ- 

 izations. That correlations of a definable nature exist, I have no longer any doubt ; and 

 after completing the partial comparisons hitherto made only between the young of the 

 Ammonites and the shells preceding them in time, I hope to show that the life of the indi- 

 vidual, so far as the shell and its internal structure indicate what its metamorphoses were, 

 displays during its rise and decline, phenomena correlative with the rise and decline of the 

 collective life of the group to which it immediately belongs. 



D'Orbigny divides the life of the individual into five periods, distinguishable from each 



l I have also undertaken, at the suggestion of Professor Brazil to be published; in this will be found the names of every 

 Agassiz, a Catalogue of the Ammonites, the first part of which new species that may be mentioned farther on. 

 is now ready (or the press and only waiting his return from 



