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 IV. Fig. 32, cl. rtn.') 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 five retinular cells. 



The retina in B. vernalis contains no other cells than the three kinds 

 already described. According to Clans (SG, 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 Branchiopodidae and Apusidse 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. 



Estheridce. — 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. Wood worth. 



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, nl. cm,). 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 five cells (Plate IV. 

 Figs. 37 and 38). In this respect it resembles the cones in Fstheria 

 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. 37 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. 300) statement that in Limnadia 

 the cone consists of four segments may be accurate, contrary to the 

 opinion of Lenz. 



