162 THE AMERICAN NATURALIST. {[VoL. XXXIII. 
determine to a large extent the distribution of this group. The 
local and seasonal occurrence of each species is given in tabulated 
form. The Cladocera reach their maximum development, as regards 
species, in September, and their minimum in January, The Ostracoda 
exhibit two maxima in March and in September, the latter being 
more marked, and two minima in August and in January. The 
Copepoda exhibit but a slight variation in the number of species 
during the year, though there is some suggestion of maxima and 
minima similar to those detected among the Ostracoda. The sea- 
sonal distribution of males and ephippial females of the Cladocera is 
given for 34 species. In this group, considered as a whole, there 
are two seasons of sexual activity, the first reaching a maximum 
in May and affecting only 10 species, and the second extending 
throughout the autumn months, culminating in October, and affecting 
30 species. Thus in the majority of the species the period of sexual 
activity is confined to the autumn months, though a small number 
is affected during both seasons, and a few exhibit but a single . 
annual period in the spring. In only a single species, Daphnia 
longispina, does sexual reproduction continue from May to October. 
Colonies of a given species found in different aquatic habitats may 
present marked differences in sexual activity, the size of the body of 
water seeming to be correlated with the variations. Thus males and 
ephippial females of Simocephalus vetulus have been found in the 
spring only, in tiny pools and ditches, and again in the fall in bodies 
of water of slightly larger size, while no trace of either sexual form 
was observed in a larger pond examined repeatedly during a period 
of three years. The author is inclined to attribute these differences 
to the direct action of the environment. 
A second paper’ by the same author deals with the biology of a 
common water-flea in an interesting way. A respiratory function is 
assigned to the anal cecum, a thin-walled triangular sac, with glan- 
dular cells in the dorsal wall. It is constantly dilating and contract- 
ing, and produces a circulation of water that suggests its respiratory 
function. As in the Cladocera generally, parthenogenesis prevails 
with alternating periods of sexual activity. The sexually mature 
female produces the so-called winter or resting eggs, which, unlike 
the parthenogenetic or summer eggs, require fertilization in order to 
develop. The ephippium, which carries the resting egg, is formed 
in Chydorus from the cast-off shell, which is somewhat thickened, 
1 Scourfield, D. J. Chydorus sphericus, 7he Annual of Microscopy (1898), 
pp- 62-67, 1 plate. 
