194 JOURNAL OF SCIENCE. 
fig. 2. Here we find a number of changes. First we see that 
only two regions of the body are present, the cephalothorax and 
the abdomen, the thorax being unrepresented. The cephalo- 
thorax is not very different from that of the protozoea. The 
abdomen is, however, very different: it is distinctly divided into 
segments, all of which are well developed ; it is tolerably thick, 
and isa much more powerful structure than the corresponding 
part of the protozoea. The muscular and usually the nervous 
system is well developed. In short, the abdomen is much more 
perfect than that of fig. 1. The locomotion of this zoea is en- 
tirely different from that of the protozoea. It does not use its 
antennee-for moving, but propels itself vigorously with powerful 
strokes of its abdomen, after the manner of the lobster; at least 
this is its motion when trying to escape danger, and that is all 
that concerns us. In correlation with this changed locomotion, 
the antennez have altered their form, and are now true sense 
organs. On the other hand, the telson has become broadened 
into a flat swimming organ. It is much broader than the rest 
of the abdomen, and is used as a paddle to augment the effects 
of the powerful strokes of the abdomen. It still retains a num- 
ber of spines, but they are usually quite small. 
A third type is the zoea of the ordinary crab. Fig. 3 is such 
a zoea. Here we see anumber of striking peculiarities. Asin the 
shrimp zoea, we find no middle body—ze., the thorax is absent. 
The abdomen is quite small, and always occupies a characteris- 
tic position. Instead of being stretched out behind the body, as 
in the shrimp zoea, it is bent under the cephalothorax as in the 
figure. Still another mode of locomotion is here found. It is 
true that it occasionally uses its tail, but its ordinary locomotion 
is neither with antennz nor abdomen, but by means of its first 
two pairs of maxillipeds. These are very long, and carry large 
numbers of swimming-hairs, and serve as oars, with which the 
zoea paddles itself along. Its motion, while swifter than that of 
the protozoea, is not so vigorous as that of the shrimp. The 
tail has become modified into a form half-way between the tails 
of the other two larve described. It is somewhat broadened, 
and probably has a slight motor function, but its chief use is 
protection (fig. 4). The most noticeable feature is the very re- 
markable cephalothorax. This is of enormous comparative size, 
entirely covering the body when the abdomen is flexed. It is 
further armed with a number (usually four) of long spines, which 
project in different directions, and are strong and sharp. No 
one can be in doubt as to the use of this arrangement. The 
large cephalothorax, with its resisting spines, serves as a protec- 
tive case for the more delicate organs within ; and, further, when 
the abdomen is flexed, the spines of the peculiar telson are placed 
in such a posi‘ion as to give additional protection, being then 
directed forwards. 
Now, is there any connection between these three forms, and 
is it possible to discover any explanation for their peculiarities ? 
In the first place, comparative embryology shows good reasons 
