SUDLER : DEVELOPMENT OF PENILIA. 
129 
but he is unable to distinguish it from its fellows after it has sub¬ 
divided into eight cells. He cannot trace it as the mother-cell of 
the genital organs, and from my work on Penilia and Lebedinsky’s 
on Daphnia it is evident that Grobben’s results are not confirmed. 
Certain facts in the development of Penilia lead me to think 
that it is a highly specialized rather than a primitive cladoceran. 
The second maxillae appear as late and disappear much sooner than 
they do in Moina. The dorsal gland is weakly developed in com¬ 
parison with other members of the group and lingeous a shorter time. 
The entoderm is nearly distinct as such from its origin and is never 
at any time intermingled with the mesoderm as it is in Moina. 
McMurrich thinks the origin of the entoderm and mesoderm 
together is the rule for the entire crustacean group. In the method 
of forming the eggs it resembles what is found in other Cladocera, 
since the egg is here apparently the survivor of four ovarian cells 
which unite to give rise to but one with little yolk. All of these 
facts indicate a specialized type. 
Penilia also presents other characteristics considered primitive for 
the Cladocera. Claus finds six pairs of thoracic appendages present 
in the metanauplius hatched from the winter egg of Leptodora 
hycdina , which he thinks must be considered the original or primi¬ 
tive number for the Cladocera. As we have seen, the presence of 
six pairs of thoracic appendages is the chief distinguishing feature 
of Penilia. If Claus’s view be the correct one, then Penilia must 
have branched off from the cladoceran stem quite early but yet 
late enough to have inherited the peculiar yet characteristic method 
of forming its eggs each from four equal ovarian cells. 
I desire to express my thanks to Dr. C. P. Sigerfoos for the 
material upon which this work was done. Also my indebtedness to 
Prof. W. K. Brooks who suggested the work and throughout its 
progress gave advice, and to whose kind interest is due much of 
whatever value my results may possess; and I also acknowledge my 
indebtedness to Dr. H. McE. Knower for his kindly criticisms. 
