APRIL 25, 1884.] 
larva cannot enter the tube. Finally the protozoea 
will be easily caught: it swims slowly, and cannot 
escape the tube; nor does it present projecting spines 
which prevent its entrance into a small orifice. 
This simple experiment teaches us four things: 1°. 
The dread of suction exhibited by all forms indicates 
that their chief enemies are small animals, largely, 
perhaps, fishes which swallow them in their widely- 
opened mouth; 2°. The behavior of the macruran zoea 
shows evidently, that, in its struggle for existence, 
it relies for its protection upon its power of flight, 
and this gives us immediately a hint as to the mean- 
ing of the broad tail; 3°. The crab zoeas rely for their 
protection, not upon flight, but upon the efficacy of 
their defensive armor, either as an actual defence, 
whose resistance baffles the jaws of the fish, or as 
an apparatus which prevents their entering the mouth 
of a small enemy (this consideration immediately 
explains the use of the excessively long spines in 
Panopeus and Porcellana, which seem to be such 
encumbrances to the freedom of the larva); 4°. The 
protozoea seems to possess none of these means for 
protection; and, indeed, in every respect the proto- 
zoea seems ill protected. Its slow, hesitating motion, 
its long weak abdomen, its long antennae with their 
numerous swimming-hairs, —all render it easily en- 
tangled by rubbish, and easily caught by any enemy. 
Taking all of these points into consideration, we 
get suggestions as to a possible explanation of the 
remarkable differences between the crab and the 
shrimp zoea, — differences which seem difficult to 
understand, since the Brachyura and Macrura are 
evidently so nearly related. All decapod larvae are 
freely swimming animals, gaining their own living 
by an active search for food: they are therefore sub- 
jected to a struggle for existence precisely similar 
to that of adult animals. The principle of natural 
selection will be as potent to select and modify them 
as it is in selecting and modifying adults. If, there- 
fore, we assume the protozoea as an original form, 
we must expect to find it in many cases highly modi- 
fied, and must expect in most larvae to find, not a 
protozoea, but a greatly different form, and one better 
adapted for the struggle for existence. Nor must we 
be surprised if the embryologist comes to the conclu- 
sion that the modified larval stages do not represent 
stages of ancestral history. 
That the protozoea larva is not well adapted for a 
struggle with numerous enemies is evident to any one 
who observes how easily it is captured. Assuming 
that this is the early larval form, we should not ex- 
pect, from what we know of the workings of nature, 
that such an evidently weak form would be preserved, 
except in isolated cases. To adapt such a larva to a 
more effective struggle, there are three inethods: the 
larvae may be largely increased in numbers, which 
would, of course, increase the chance of the species 
for survival; or they may develop powers of flight, 
which will enable them to escape their enemies; or 
the larvae may develop some sort of defensive armor, 
which will enable them passively to resist all ordinary 
attacks. Abundant examples of each of these methods 
may be found in almost any group of the animal king- 
SCIENCE. 
O15 
dom, but probably no better instances than the larvae 
in question; and this is all the more interesting, since 
it shows that some of the principles affecting adults 
also in a similar way have their influence on larvae. 
With these points in mind, itis possible to explain all 
of the important differences between the protozoea 
and the two zoea types. 
What explanation can we find for the shortened 
body? Two explanations for this can be found, both 
of which probably had their influence. The posses- 
sion of such a long, weak, almost functionless hind- 
body as is found in the protozoea is certainly caleu- 
lated to render its possessor a more easy prey to 
enemies than it would be were the body more com- 
pact. The shortening may therefore be simply a pro- 
tective measure. Or a second principle has probably 
had even more influence. There is good reason for 
believing that the amount of energy of a developing 
animal is limited, and, if expended in one direction, 
cannot be employed in a second. If, for example, 
a child over-develops its brain, its body is sure to 
suffer. Now, this principle has had a similar effect in 
our larvae. In the protozoea the energy of develop- 
ment is evenly distributed to all parts of the body. 
The result is, that we find here a larva with almost 
all of the body present, but in a low state of develop- 
ment: the larva is consequently comparatively weak. 
If, however, the development of a part of the body 
should be postponed, the parts which did develop 
could reach a greater state of perfection, since the 
whole energy of development could be directly turned 
toward their perfection. In all existing zoeas the 
development of the thorax has been thus postponed. 
The zoeas are found, therefore, to be much more 
vigorous than the protozoeas, their muscular and 
nervous system is better developed, and they are in 
all respects more fitted for an active struggle for 
existence; and this applies equally well to the ma- 
cruran or the crab zoea, and will assist in accounting 
for the absence of a thorax in the two forms, —a 
point which seemed a great difficulty to Balfour. 
In other respects the crab and the shrimp zoea have 
taken two different lines. The macruran type has 
become modified for its struggle by acquiring great 
powers of flight: we find its body, therefore, long and 
slim; but, unlike the protozoea, it is very powerful, has 
well-developed muscles, and a broad, paddle-like tail, 
which, with the assistance of the powerful abdomen, 
forms an effective organ of flight. Every thing which 
might impede its motion has disappeared. The an- 
tennae are small, and the other appendages are such 
as to present no hinderances. The whole body has 
become adapted to its swift motion. 
On the other hand, the crab zoea has taken a differ- 
ent line, and has developed, instead of a power of 
flight, a defensive armor. Its cephalothorax has en- 
larged, has become strong, and has developed a num- 
ber of defensive spines, whose use has already been 
noticed. Its tail, not particularly needed for swim- 
ming, has not developed into a broad plate, but has 
become an augmentation of the defensive armor by 
the form and position of its spines. Some species 
have carried this line of development still farther, 
