CRUSTACEA — DEC A POD A. 
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the four lateral leaves of the caudal fan, and the proportion ably shorter 
and thicker thoracic and thinner caudal parts, is to be remarked. In 
the lobster, the posterior feet are wanting, and the ambulatory feet 
have appendages for the purpose of swimming ; the young of Pagurus, 
Oalathea, Crangon, Palmmon, and Hyas, have, when they leave the 
egg, neither abdominal members, posterior legs, nor gills ; the propor- 
tions of the cephalothorax and abdomen are quite ditferent ; the former 
has often processes which afterwards become contracted. — 3. The most 
essential difference of form between the young and adult is, that the 
thoracic portion of the anterior part of the abdomen, during growth, is 
retarded more or less in its development, and with it, also, that of the 
ambulatory legs and gills. These parts are farthest advanced in their 
development in the river cray-fish ; whilst in Pagurus, Galathea, &c., 
they are altogether wanting. — 4. The equalization of these differences 
occurs, in the lower grade, by alterations of the proportions (as is the 
case in the river cray-fish), in the higher, by the transformation of 
individual parts, as well as of the whole body. — 5. The varieties of form 
between the young and adult individuals of the same species are greater, 
in an inverse ratio, to the degree of development in which the embryo 
leaves the egg. — 6. The following are the causes of the metamorphosis 
of the Decapod a : — a. Portions of the body are afterwards formed: 
h. Those already present increase in size, without essential alteration of 
form : c. Or they, at the same time, alter in form : d. On the contrary, 
other parts are retarded in their growth : e. Others, again, are entirely 
cast off. — 7. It is worthy of attention, that the river cray-fish, when it 
comes from the egg, can only creep, while the young of the sea Decapoda 
swim. — 8. With regard to the limbs of the Decapoda, the rule in 
general seems to be, that they divide into two branches, and as develop- 
ment proceeds, each branch takes a different form and direction. (If 
we find in Decapoda and Stomapoda, the gill-bearing members simple, 
but those without giUs cleft, the conclusion is, that the gills correspond 
to the second branch of the limbs. But, according to Rathke’s obser- 
vations on the lobster, the case is different ; the gills are formed by 
themselves, and one branch of the ambulatory feet becomes wholly obli- 
terated). — 9. In some Crustacea, the cleft feet remain during life {My sis). 
— 10. However much the young lobsters resemble Schizopoda, particu- 
larly in their organs of motion, the development in these, in general, 
goes upon quite a different plan. — 11. All the Amphipoda, without ex- 
ception, appear very like their parents at the time they leave the egg ; 
in other orders, particularly the Decapoda and Isopoda, great differences 
exist. 
H. Goodsir (James. Edinb. New Philos. Journ. xxiii. p. 181) has con- 
firmed, in opposition to Westwood, Thompson’s discovery of a change of 
the Decapoda, in Carcinus jncenas and Pagurus hernkardus, without 
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