290 



ROBERT TRACY JACKSON ON THE 



of growth which immediately succeeds it (see figures referred to iu the summary). In 

 view of the fact that it is the close of the embryonic shell period and is expressible as a 

 completed protoconch or prodissoconch, I do not think it is necessary to name the pe- 

 riod of growth. The period in Pelecypods and Gasteropods, where both anatomy and 

 shell are tnown in many genera, is referable in origin to the adults of known allied 

 forms. 



In the early period of growth of the true shell, which succeeds the first formed embry- 

 onic shell, stages have been found which are referable to adnlt ancestral forms. Pro- 

 fessor Hyatt stndied these stages in Cephalopods and named them the silphologic (grub) 

 stages. In a footnote to this paper^ he has given up the use of the term silphologic and 

 supplied in place of it the woi'd nepionic.^ 



My investigations of young Pelecyi3ods and Grasteropods, and also stiulies of pub- 

 lished figures of Scaphopods, lead to the conclusion that nepionic stages in molluscs have 

 characters referable to ancestiy in most cases quite close to those naturally grouped as 

 near allies of the adult. In the nepionic period (see figures referred in the Summarjr), 

 it is quite common to find a shell with characters almost inseparable from those of the 

 adult, or at least so near that the genus can be correctly determined. In other cases, 

 on the contrary, wide ditferences from the adult condition may be observed at this earl}'' 

 stage. 



Periods later than neinonic are named by Professor Hyatt successively as nealogic 

 (yoiithful), ephebolic (adult), and geratologic (old age) stages. 



We will now give a brief summary of the stages of growth and decline as applied to 

 the mollusca, with figures or references to known forms which may be used as types il- 

 lusti'ative of the various stages. The illustration of the stages is a new feature, not given 

 in Professor Hyatt's paper. Definitions in this summary are taken from that author's 

 paper with the exception of such alterations as are suggested in the foregoing pages. 



A classification of stages of growth and decline, in its apjilication to the various groups 

 of the animal kingdom, must be sei)arately worked out for each group. It is believed, 

 however, that the following classification contains the essential features necessary for 

 such applications. I^ew terms may be needed in some groups, but those here used will 

 probably be applicable to the principal periods of growth in all animals. 



Protembryo. The ovum and stages of segmentation of the Qg,^ preceding the forma- 

 tion of the blastula cavity. Examples: figs. 10-15, p. 295, of develojiing Ostrea, also 

 Professor Bi'ooks' figures 1-25 of a developing oyster and Dr. Patten's figures 2-11 of 

 developing Patella. 



' The use of the Tvonl "silphologic" d?rivL'd from 

 alkiPTi a gi'iih, when applied to the younjier stages of all 

 animals is likely to lead to confusion of ideas. I have as- 

 certained, by recent studies of insects, the truth of Brau- 

 er"s opinion, that the gruhs, maggots and caterpillars 

 common among insects are not normal progressive stages 

 of development, such as are most common in other types 

 of animals; hut are, strictly speaking, cases of retrogres- 

 sive or more or less degraded specializations. I desire, 

 therefore, to give up the use as a general term of the word 

 SUphologic ^nA propose in its place Nepionic, or young an- 



imal, from .Vry-!i)j, an infant. This word also is more in 

 accord with other terms of tlie nom;'nclatnre proposed in 

 my paper on "Values in Clas.sification of the Stages of 

 Growth and Decline," Proc. Bost. Soc. Nat. Hist. Vol. 

 XXIII, 1888. Signed, Alpheus Hyatt. 



* The word nepionic is particularly suitable for early 

 yet post embryonic stages. Basld?s its meaning of infant, 

 applied to the young of man, I am told by Mr. Ernest 

 Jackson that it is used twice in the Iliad, of young ani- 

 mals, and by Theo])hrastus of young plants. 



