MUSEUM OF COMPARATIVE ZOOLOGY. 75 
sibly he was influenced when counting the nuclei by his belief that the 
number four was characteristic of many structures in the ommatidium, 
In Branchipus vernalis (Plate 1V. Fig. 32, c/. rtz.') the retinula contains 
five cells. 
The rhabdome in Apus is short; in Branchipus (Fig. 30, rhb.) it is 
relatively long. In transverse section (Fig. 32, rhb.) it is circular, or at 
times squarish, but never pentagonal, as might be expected from the 
fact that it is surrounded by jive retinular cells. 
The retina in B, vernalis contains no other cells than the three kinds 
already described. According to Claus (86, p. 319), blood corpuscles 
may make their way into the base of the retina of B. torticornis. 
From the preceding account, the number of cells in the ommatidia of 
the Branchiopodide and Apuside can be stated as follows: cells of the 
corneal hypodermis, usually two, possibly variable in number in some 
species ; cone cells, four; retinular cells, five. In Branchipus torticornis 
the interommatidial space may contain blood corpuscles. 
Estheride.—The species which I studied as a representative of this 
family was Limnadia Agassizii, Packard. This species can usually be 
obtained in great abundance during summer in small fresh-water pools 
in the neighborhood of Wood’s Holl, Mass., where my material was 
kindly collected for me by Mr. W. M. Woodworth. 
The external surface of the retina in Limnadia, as I have mentioned 
in my account of the general structure of the eye in this genus, is cov- 
ered with an extremely delicate corneal cuticula. This cuticula does 
not show the least trace of facets. 
Immediately below the corneal cuticula are numbers of small nuclei 
(Plate IV. Fig. 37, nd. ern.). These, from their position, are probably 
to be regarded as the nuclei of the corneal hypodermis. They are not 
regularly arranged, and, although they sometimes lie between the cu- 
ticula and the distal end of a cone, they more frequently occur next 
to the cuticula in the spaces between the cones. 
As a rule, each cone in Limnadia is composed of jive cells (Plate IV. 
Figs. 37:and 38). In this respect it resembles the cones in Estheria 
californica and E. tetracera described by Lenz (’77, p. 30). In Lim- 
nadia Agassizii, however, cones composed of four cells are not infre- 
quently met with (compare Figs. 387 and 38). Grube’s (’65, p. 208) 
observation that the cone in Estheria is composed of two segments is 
probably erroneous, but Claus’s (’72, p. 360) statement that in Limnadia 
the cone consists of four segments may be accurate, contrary to the 
opinion of Lenz. 
