No. 2.] ANURIDA MARITIMA. 261 
Leaving the podurans, there are points of interest to be 
found in connection with the higher groups. Wheeler ('93) 
discusses a curious structure found in Xiphidium, which he 
calls the “indusium.” It appears at the same time as the germ 
band, or a little later, and is ventrally placed on the long oval 
egg, just in front of the head. It remains unchanged for some 
time, usually separated from the head, but sometimes connected 
by a small string of cells. Ultimately by proliferation it forms 
an envelope, pushing its way between the serosa and the yolk, 
and finally becomes an inner membrane next to the yolk, and 
only separated from the embryo by the amnion. Strange to 
say, this organ forms for itself an amnion, which spreads round 
the egg and is recognized as the outer “indusium.” The author 
homologizes this with the poduran “ micropyle,’ and seconds 
the previous suggestion that the latter is truly homologous with 
the “dorsal organ,”’ as found in some groups of the Crustacea. 
Among the Crustacea there are found curious intermediate 
structures. Bobretsky (74), in his studies of Oniscus, which 
have been corroborated in part by Nusbaum ('86), discusses the 
so-called “‘ primitive cumulus” or “dorsal organ.” Its origin 
as described is similar to that of the precephalic organ in 
Anurida both in manner and in time. Excepting in the dorsal 
half of the embryo the two germ layers are distinct. After 
remaining stationary for a long time the cells increase in 
number and spread out over the dorsal part of the embryo as 
acap. This cap is connected with a thin membrane that the 
author calls a larval skin. At the greatest development of 
this organ it remains as a saddle-shaped cloak composed of a 
single layer of cells. 
In embryos of two species of Idotea found at Woods Holl, 
a structure similar in its early stages to that of Anurida was 
found, but it was paired, one small organ being placed on each 
side of the middle line. This resembled closely Nusbaum’s 
(87) figures of Mysis. 
To make a graded series between Anurida, Oniscus, and 
Xiphidium is easy. In the first the organ is large, active, and 
functional in very early stages; it later begins to degenerate 
and assumes certain secondary characters, as, for example, the 
