JIEMOlliS OF THE NATIONAL ACADEMY OE SCIENCES. 27 



grooves jiassiiig down tlie sliaft from tlie riDtclies between the teetli. They occur not only on the 

 back and sides of the body segments, but also on the sides of the abdciminal legs. The occurrence 

 of such hairs in this genus is interesting from the fact that they have not yet been observed in 

 Arctians, to which this moth has been referred, nor in the Noctuida-, among which it should be 

 placed, since no Arctians have when hatched smooth glandular hairs. 



1 



IV.— ON THIv INCONGRUENCE BETWEEN THE LARVAL AM) ADULT CHARACTERS OF NOTODONTIAXS. 



As is well known to zoologists, from the writings of Fritz Miiller and later students, in groups 

 of animals which generally undergo a nietam()ri)hosis, two or more species of the same geTuisniay 

 differ remarkably in respect to their early life, one species jjassing through a complicated meta- 

 morphosis while a closely allied form has a direct development, hatching in the form of the adult. 

 The embryo, however, in the latter case rapidly passes through a series of changes, constituting 

 a premature, abbreviated, or condensed metamorphosis, epitomizing the ordinary early stage of its 

 metamorphic allies. Thus the lobster differs from the other marine macruran Crustacea in having a 

 condensed metamoi'phosis before hatching from the egg, rapidly i)assing through a nauplius and 

 a zoi'a phase. Jt is s) with some crabs. All the fresh- water Decapoda, notably the crayfish, have 

 no postembryonic metamorphosis. The fact that the embryo exhibits a condensed metamorphosis 

 shows their origin from metamorphic forms. 



These are ]ierhai)s the most remarkable cases of incongruence between what may be closely 

 allied genera and e\en species. 



Also two allied species of Gammarus may differ in toto as regards the mode of segmentation 

 of the yolk, total cleavage occurring iu one marine species {G. Jociista) and partial or peripheral 

 cleavage in two fresh water forms {(f. piile.r and JJiiriatilis). 



Examples of such great divergences in larval or early life, or in the condition in whicli the 

 animal is hatched, in species closely similar in adult life, are not uncommon in worms, Echinoderms, 

 Molluscs, Crustacea, besides insects, and the phenomenon is with little doubt due to the changed 

 conditions of the environments to which forms with such exceptional modes of development have 

 been exposed. 



The principle, then, of divergence or incongruence of larval characters in forms whose adults 

 are closely allied has been established in the lower classes of JFetazoa. The most remarkable 

 and ])uzzling case, perhaps, is that of Balanoglossus, whose Tornaria larva is so much like that of 

 Echinoderms, while the adult is a protochordate animal. 



As a matter of fact this does not affect the classification of these animals. Zoologists have 

 not thrown forms with a direct development into distinct groups where the adults have not 

 shown any differences; at the same time no one would unite the two species recognized as such 

 which i^resented no easily observed differences if one had a direct and the other a metamorphic 

 development. In the present state of our knowledge it may be well to at least provisionally 

 mark the differences between the two forms, so divergent in their early life, by giving them 

 distinct names, and thus emphasizing the fact that of the two closely allied forms one has 

 diverged from the other through having been subjected to a different set of external influences, 

 whatever such conditions may have been. 



Systematic zoology has undergone within the last thirty years an entire change. Our present 

 systems of classification are now attempts to arrange animals in the order of their probable 

 a])pearance, i. e., phylogenetically, and as the subject is yet iu its infancy, and our attempts 

 provisional and tentative, we are obliged to give great weight to any differences iu the larval 

 conditions of animals with a metamorphosis, because such differences were undoubteiily dueto 

 differences in the environments of their jiarents. Indeed if it had not been owing to changes in 

 the physical and biological environment, aninmls would lU'ver have risen beyond the dead level 

 of the lowest Protozoa. 



Such reflections as these and a knowledge of the mode of development of the lower classes 

 of Invertebrates are all-important to the students of insects, especially of the metamorphic orders, 



Tl. XXXVII. fig. 12. Glandular liairs of Certitosia tricolor, a, I'roiii the second thoracic and first ahdoniinal seg- 

 ment ; b, those on the first and second abdominal legs. 



