Vol. II., No. 5, SEPTEMBER, 1884. 
EVOLUTION OF THE<DECAPOD ZOEA. 
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(Science, Vol. III... p. 513, April, 1884.) 
Principles applicable to adults are often equally applicable to 
larve. In the discussion of natural selection most writers have con- 
fined themselves to adult animals and their reaction upon environ- 
ment. There is no reason, however, why the principle should 
not be extended to include larval forms ; and, indeed, toa slight 
extent this has already been done. Weismann’s “ Theory of 
Descent” proceeds upon this line, and indicates some of the im- 
portant results which may arise from such research. 
Crustacean larve offer particularly good opportunity for work 
in this direction. They are abundant, are easily obtained, and 
readily studied. They present great varieties of form, which are 
frequently not in any degree related to the adult characteristics. 
Indeed, crustacean larvee seem almost like a distinct group of 
animals, and may be studied as such, with the extra advantage 
that they are highly variable, and tmdergo rapid metamorphosis. 
Some of the possibilities of such research may be seen by a short 
consideration of the different forms of decapod zoea. 
- To make the subject clear it will be necessary to give a brief 
description of three types of decapod larve, confining ourselves, 
however, only to such points as particularly concern us here. The 
first is the type, which is undoubtedly the oldest, known as the Pro- 
tozoea. It is a comparatively rare form, being found in a few 
macruran species (Peneus, Lucifer, Euphausia). Fig. 1 represents 
such a larva. As far as concerns us, the peculiarities are these :— 
The long body consists of a large cephalothorax,a more or lesscom- 
plete thorax, and an abdomen. The important point is, that all the 
regions of the body are represented. Whenviewed from above, 
the part of the body composed of thorax and abdomen is seen 
to be very slender and weak, and to extend for a long distance 
backwards. A second important point is the method of locomo- 
tion ; unlike all other forms, the antenne, instead of being sen- 
sory organs, are used in locomotion. They are large, :and 
covered with swimming-hairs, which convert them into paddles ; 
and, by moving them to and fro, the protozoea slowly propels 
itself by a series of jerks through the water. The telson is a 
third important feature; it is small, being in our figure no broader 
than the abdomen ; it is usually forked, and carries a number of 
long spines (typically seven, though the number varies); it is not 
a swimming organ—a point of particular interest. One other 
feature must be mentioned—the usual, though not universal, 
absence of protective spines. 
A second type.is that of the ordinary macruran zoea—e.g., 
the larva of the common shrimp. Such a zoeais represented in 
