SUDLER : DEVELOPMENT OF PENILIA. 
117 
derm, and the rest to either side of them are mesoderm. As has 
been shown, the separation in Penilia is much earlier. The facts 
observed in Penilia correspond to those given by Lebedinsky for 
Daphnia similis, where the mesoderm spreads forward as two sym¬ 
metrical rows from the gastrula invagination. At a later stage in 
Penilia (Figs. 17-23) the mesoderm has grown slowly forward and 
together at the mid-ventral line, making the outer wall two-layered 
with the exception of the dorsal side where only ectoderm exists. 
The gastrula mouth gradually becomes smaller, and by the time 
the second antennae show it has disappeared almost completely, 
leaving its position marked only by the entodermal rod, which soon 
breaks away, making it impossible to locate the spot where the 
anus breaks through. Grobben concludes that the mouth of the 
adult Moina arises directly from the gastrula mouth of the larva. 
This derivation of the mouth is impossible in Penilia, as will be 
seen later. In fact, it is the rule throughout the Crustacea for the 
anus and not the mouth to be formed from or near the position 
occupied by the gastrula mouth. 
The derivation of the anus in Penilia is open to question. The 
anus appears in the middle line and in the same or immediate 
vicinity as that occupied before by the gastrula mouth. The anus 
may be formed de novo slightly above or below this point. The 
gastrula mouth completely disappears, and so it is impossible to say 
•whether it ever reopens. 
The Appendages and Change in the Position of the 
Embryos. 
About this time a change in the outward form of the embryos 
makes itself apparent. The gastrula mouth still lies at the posterior 
and outer angle of each egg in reference to the mother and in the 
middle line in reference to the embryo. The anterior end flattens 
itself very slightly dorso-ventrally and becomes broader than the 
posterior. Two pairs of prominences appear anteriorly; the ante¬ 
rior of which are very small and weakly developed, frequently noth¬ 
ing more than the smallest angles on this end of the embryo. The 
more posterior and larger are more sharply pronounced. They 
project backwards on either side from the anterior flattened area 
and make the flattening effect much more pronounced. Both pairs 
of these prominences lie in a plane perpendicular to one cutting the 
