EVOLUTION OF THE DECAPOD ZOEA, 195 
for believing that the first type, protozoea, is the oldest, and that 
the others are derived from this form. The evidence cannot be 
here deduced, but may be found by referring to Claus, Brooks, or 
Balfour. Assuming then this to be the case, the question re- 
solves itself into the simpler one—What caused the protozoea 
to undergo changes which converted it into the remarkable zoea 
form? 
A simple experiment, easily performed by anyone at the sea- 
shore, suggests an answer. The experiment is simply to endea- 
vour to catch a specimen of each of these types of larvee with a 
moderately small dipping-tube. It will be noticed that all of the 
larvee seem to have a dread of the suction which is produced by 
the tube, and all will swim away from it unless it be too strong, 
It will be further seen that it is next to impossible to catch the 
shrimp zoea. He darts away with the vigorous strokes of his tail, 
and unless the fisherman is very quick heis gone. Some of the crab 
zoeas will be easily caught, but they will be seen upon examina- 
tion to have doubled themselves up into as compact a mass as 
possible, with all their spines projecting, and consequently in 
position to offer the greatest defence against enemies. Other 
crab zoeas will be found not so easily caught. If the zoea 
fished for be of the species figured, or still better be the larva of 
Porcellana, and the dipping-tube be small, it will be found im- 
possible to catch it. The long spines project so far in different 
directions that the larva cannot enter the tube. Finally the 
protozea 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 experi- 
ment teaches us four things :—1st. The dread of suction exhibi- 
ted by all forms indicates that their chief enemies are small 
animals, largely perhaps fishes which swallow them in their 
widely-opened mouth; 2nd. The behaviour 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 im- 
mediately a hint as to the meaning of the broad tail ; 3rd. 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) ; 4th. The protozoea 
seems to possess none of these means for protection ; and, indeed, 
in every respect the protozoea seems ill protected. Its slow, 
hesitating motion, its long weak abdomen, its long antennze 
with their numerous swimming-hairs,all render it easily entangled 
by rubbish, and easily caught by any enemy. 
Taking all these points into consideration, we get suggestions 
as to a possible explanation of the remarkable differences be- 
tween the crab and the shrimp zoea—differences which seem 
difficult to understand since the Brachyura and Macrura are 
