Ig2 Trans. Acad. Set. of St. Louis. 



All who have had a first-hand acquaintance with Daph- 

 nia hyalina have noticed the marked morphological differ- 

 ence of individuals captured from the same pond. In 

 some the head bears a spine, in others it does not ; in some 

 the dorsal spine is short, in others it is long; in some 

 cases the spine is at the dorso-caudal angle of the shell, 

 in others it is lower down. Heretofore, so far as my 

 knowledge goes, these forms have been considered indi- 

 vidual peculiarities, distinct varieties, or even distinct 

 species. The relative length of the spine is apparently 

 an individual matter, at least its relative size is not deter- 

 mined by the age of the animal. In the forms that are 

 found around Augusta, the other differences are differ- 

 entiations due to age. In the young embryo in the brood 

 pouch, the caudal spine is so bent down and appressed 

 against the body that the posterior border of the body 

 appears to be rounded (PL XXXVII. f. 1) or slightly 

 pointed (PL XXXVII. f. 2) and the head is spineless. 

 The free swimming but immature form has a long caudo- 

 dorsal spine and a short, but prominent, head spine (PL 

 XXXVII. f. 3) ; in this stage the body is relatively quite 

 long. Between now and the breeding stage, the body 

 becomes relatively much higher; otherwise there is no 

 marked change, except, perhaps, a slight shortening of 

 the spine on the head (PL XXXVII. f. 4-5). When the 

 animal begins to breed, the long spine on the posterior 

 border progressively retreats ventrad; this is due to the 

 enormous development of the brood sac (the old female 

 bearing fully three times as many eggs as the youngest 

 breeders— PL XXXVII. f. 6-8). In old forms the spine 

 on the head disappears entirely (PL XXXVII. f. 8). 



bosmina Baird. 



25. BOSMINA. LONGIKOSTEIS 0. F. M. 



Bosmina longirostris, Birge, '78, p. 15. 



Bosmina longirostris, Herrick & Turner, '95, p. 207; pi. XLV., fig. 

 2; pi. LXV., fig. 2. 



This form was abundant in the temporary portions of 

 a large no-outlet pond, at a water temperature of 21° C. ; 



