SUDLER : DEVELOPMENT OF PENILIA. 
123 
portion represents the beginning of the thoracic appendages. Near 
the center of these squarish limb-rudiments which represent the 
basopodite a portion becomes more prominent, projecting directly 
downward. This eminence is the beginning of the endopodite and 
first shows itself in the most anterior thoracic appendage, although 
the difference in the time of their development is not so marked as 
that of the first appearance of the thoracic segmentation. The endo¬ 
podite grows ventralwards until we have it standing nearly at right 
angles with the exopodite. The free ends of these bend more 
toward the mid-line as they grow older and the shell grows back 
covering them. All of the thoracic appendages are bifurcated in 
the adult. The third of the series of thoracic appendages becomes 
the largest in the adult and they grade off from it smaller in either 
direction. The sixth or last pair is the smallest but preserves the 
structure typical of them all. 
The presence of six pairs of thoracic appendages is the chief dis¬ 
tinction of the genus Penilia, and Claus states in reference to the 
presence of six thoracic appendages in a larva of Leptodora hya- 
lina that we must consider this to be the original number for the 
Cladocera. 
The Shell. 
The shell of Penilia appears as a lobed fold of ectoderm on either 
side just above the maxillary region at a stage intermediate be¬ 
tween those shown in Figs. 1, 5, 28. The integument here rises 
up into a fold projecting posteriorly. This fold is lobed, as seen 
from a dorsal view, with the apex of the angular area dividing 
the two shell-rudiments directed toward the head of the embryo. 
This is shown by the most anterior embryo of Fig. 1. On either 
side this fold runs well down to the ventral surface. Seen from 
the ventral side when young, it resembles an appendage to a 
considerable extent, since it is about the same width and com¬ 
posed of two layers of cells and lies in the same general position 
as that assumed by a thoracic appendage. These saddle-folds 
grow back gradually on either side, covering the body and appen¬ 
dages as we have it in the adult. The rate of growth and shapes 
can be seen in Figs. 1, 5, 6, 28, and 29. 
