EVOLUTION OF THE DECAPOD ZOEA. LO? 
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 macruran 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, there- 
fore, 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. Everything which might impede its motion has 
disappeared. The antennee are small, and the other appendages 
are such as to present no hindrances. The whole body has 
become adapted to its swift motion. 
On the other hand, the crab zoea has taken a different line, 
and has developed, instead of a power of flight, a defensive armor. 
Its cephalothorax has enlarged, has become strong, and has de- 
veloped a number of defensive spines, whose use has already 
been noticed. Its tail, not particularly needed for swimming, 
has not developed into a broad plate, but has become an aug- 
mentation of the defensive armor by the form and position of its 
spines. Some species have carried this line of development stili 
farther, and are provided with enormously long spines, many 
times the length of the body, which effectually prevent their 
being swallowed by small animals. The development of the 
spinous protection would seem to be correlated with the absence 
of a swimming tail. Some species (Pzxnotheres, Tatuira) which 
do not possess any of these, show a tendency toward a modifica- 
tion of the telson, which has in these cases become quite broad 
and flat. 
We may assume then that at one time the decapods, or the 
stem from which they arose, universally possessed a larval stage 
somewhat similar to the form known asa protozoea. As the 
struggle for existence became more and more severe among 
the crustacea, modifications arose which took two directions. 
The adults became changed ; and there arose in this way the dif- 
ferent types which we know as Anomura, Brachyura, and Macrura. 
But, at the same time, natural selection had its influence upon 
the adult. The larve, therefore, also became slowly modified 
for their own protection; and from the protozoea arose the 
zoea types with their infinite variety. It is quite evident that 
these changes may take place in the larvae without materially 
affecting the adult, for the circumstances bringing them about 
influence the larve alone. Still it is probable that habits and 
forms of the adult may have some influence upon the general 
shape of the larvee. The larva must eventually transform itself 
into the adult, and the more nearly it approaches the adult form 
