23S TENTH 1.NKUAL REPORT 



group but is a very divergent membei' of it, worthy, probably, of form- 

 ing a distinct section or sab-family. The most remarkable feature is 

 one which appears in a comparatively earl}^ embryonic period and, in 

 some cases, nearly disappears in later life This is the development of 

 a long spine from the dorsal, posterior end of the shell. This is the 

 real diagnostic test and has not yet been incorporated into the defini- 

 tion of the genus The occurrence of a crista is more variable ap- 

 parently, but may be of some importance. 



The following is suggested as a revision of the diagnosis : 



Shell more or less oval or sub-quadrate and reticulate; head rounded 

 anteriorly, but sometimes with a crest, prolonged below into a beak 

 which is truncate posteriorly and bears the antennae near the apex; 

 upper dorsal corner of shell in young of both sexes and mature males 

 prolonged into a long spine; the macula nigra is present but not 

 always pigmented; the post-abdomen spined behind; opening of rectum 

 at the end. 



The female with two age-forms (heterogenetic and dimorphic); the 

 second form frequently scarcely spined; antennae small, not movable, 

 furnished with sense-hairs; ephippium with two ova, separable from 

 remainder of shell along the latero-median suture; the brood-cavity 

 closed by more than two unequal processes of the abdomen. 



Male with long movable (almost two-jointed) antennae furnished 

 with prehensile stylus; first foot bearing a curved claw; swimming an- 

 tennae very long; vas deferens opening at the end of post-abdomen. 

 Embryo with second antennae palpate; a curved appendage to shell 

 which becomes the spine of adult. 



DAPHNIA PULEX. 



This species is mentioned here simply to remark concerning D. 

 pulex, var denticulata of Birge, (Notes on Cladocera, p. 11, plate I, fig. 

 11 J that the European as well as all the American specimens of D. 

 pulex, have a fine series of spines on the claws of the post-abdomen. 

 A glance at Tafel XII, fig. 39, of the Zeitschrift fiir Wiss. Zool. Bd- 

 XXXIII, with Weismann's plate of the end of the abdomen of this 

 species, is sufficient evidence of this fact, though as the animal is a 

 ]iiale and quite young, the spines are less evident; moreover the 

 number of caudal teeth is known to be variable with age. Some 

 other peculiarity must be found to give this varietal distinction 

 validity. 



