382 Heterogenetic Development in Diaptornus, [April, 



stages in adult Cyclopidae. I am now able to set the matter at 

 rest with reference to these two forms at least. Having had 

 occasion to collect fresh-water animals through the entire length 

 of the Mississippi valley from Lake Superior to the Gulf of Mex- 

 ico, many hundreds of specimens of Diaptomus have been exam- 

 ined in the most diverse localities. If it were permissible to 

 establish a species upon slight variations in structure, numbers of 

 them might be distinguished. However the following facts debar 

 me from attempting it : At Decatur, Ky., a series of small pools 

 in various stages of stagnation, furnished an opportunity for study- 

 ing the variations due to age and conditions of the water. 



It is. remarkable that in such small bodies of water only one 

 stage may be present in one, while the next, a few feet away, may 

 offer another. 



In the same localities the various stages of a Phyllopod could 

 be studied in the same way. 



The normal D. castor^ like Minnesota specimens in its various 

 stages, occurs in some pools, in others, a few steps away, occurred 

 a larger form, at a glance distinguishable from the above by the 

 short antennae and stylets, and the structure of the fifth feet. I 

 was, however, struck by the fact that all these specimens were im- 

 mature (though nearly as large as D. giganteiis), and unaccom- 

 panied by the adult stages. Figs, i, 2, 3 and 9, Plate v, show 

 some of the peculiarities of the normal D. castor \ \a and (^b show 

 the effect of senility on fifth feet and antennae. Figs 4, 5, 6 and 

 9 show the corresponding parts in the enlarged form. 



Further study showed me that the difference distinguishing the 

 second from the first forms, saving the compact build of the for- 

 mer, are just those found in young of castor. It then only 

 remained to find the specimens in the process of molting with the 

 combined characters of both (Fig. 8) to convince one that the en- 

 larged form is really identical with castor, but by favorable circum- 

 stances enormously developed. 



Differentiation takes place before the mature stage is reached, in 

 the same way that tadpoles wintered over are greatly enlarged. 



I have found the typical D. sajtgicineus of Forbes in Alabama. 

 This form has some peculiarities to distinguish it from the com- 

 mon var. of D. castor. 



In view of the facts now known regarding the development of 

 Diaptonms, we may safely say that D. stagnalis Forbes, is an en- 



