570 THE AMERICAN NATURALIST [Vol. LVI 



tor or factors in the environment, although nothing definite is 

 known as to what these factors were. 



A peculiar structural feature of the heads of the young eye- 

 less individuals suggested the possible manner in which eyeless- 

 ness came about in these cases. When young, the seven eyeless 

 M&ina macrocopa had on the anterior head margin a small 

 nodule or excrescence which, though not so conspicuous at later 

 stages, yet in most cases persisted through several moults. In 

 each of these eyeless individuals the optic ganglion was reduced 

 or lacking, and the margin of the head was readjusted to com- 

 pensate for the reduced and missing organs. Substantially the 

 same structural conditions were found with the two eyeless 

 Moina rectirostris, absence or reduction of optic ganglia, the 

 shortening of the head margin and the occurrence of a small 

 bit of apparently necrotic material attached to the front of the 



It seems possible that this apparent exudate on the heads of 

 the eyeless individuals really represented an aborted or necrotic 

 portion of the embryo which included the primordium of the 

 missing parts.^ 



The fourth occurrence of eyeless Cladocera (the eleventh eye- 

 less individual seen) was June 26 in a crowded bottle of Moina 

 macrocopa. In addition to the lack of eye and of optic ganglion, 

 the brain proper was reduced in size. This animal was not ex- 

 amined until mature and an excrescence on the head, if present 

 in the young animal, had by that time disappeared. This indi- 

 vidual swam in small circles, although its swimming organs ap- 

 peared entirely normal. It died after producing two broods 

 (10 females and 12 males) of normal young. 



The occurrences of eyeless Cladocera have included three 

 species, eleven individuals and four different time periods. The 

 last three occurrences, and probably the first one, were in 

 crowded bottles, suggesting environmental factors as causative 



