290 



ROBERT TRACY JACKSON ON THE 



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

 view of the fact tliat it is the close of the embryonic shell pei-iod and is expressible as a 

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

 riod of growth. Tlie ])eriod in Pelecypods and Gasteropods, where both anatomy and 

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

 forms. 



In the early period of growth of the ti'ue shell, which succeeds the first formed embry- 

 onic shell, stages have been found Avhicli are referable to adult ancestral forms. Pro- 

 fessor Hyatt studied 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 word nepionic.^ 



My investigations of young Pelecypods and Gasteropods, and also studies of pub- 

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

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

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

 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 differences from the adult condition may be observed at tins early 



stage. 



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

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



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

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

 lustrative 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 application to the various groups 

 of the animal Ivingdom, must be separately worked out for each grotip. It is believed, 

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

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

 probably be applicable to the principal pei'iods of growth in all animals. 



Protemhryo. The ovum and stages of segmentation of the e^g, preceding the forma- 

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

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

 developing Patella. 



' The use of the word "silpholosic" derived from 

 nlXari a grub, when applied to tlie younger stages of all 

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

 certained, hy recent studies of'insects, the truth of Bran- 

 er's opinion, that the grubs, maggots and caterpillars 

 common among insects are not normal progressive stages 

 of development, such as are most common in other types 

 of animals; but 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 

 Silphologic and propose in its place Nrpioiiie, or young an- 



imal, from JVrj-m^, an infant. This word also is more in 

 accord with other terms of the nomsnclature proposed in 

 my paper on "Values in Classification of the Stages of 

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

 xxiii, 1888. Signed, ALrnEus Hyatt. 



" The word nepionic is particularly suitalile for early 

 yet post embryonic stages. Besides 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 Theophrastns of young plants. 



