284 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
limbs, complete evidence of which was not furnished by the 
development of the crayfish. In this crustacean, in fact, 
it would appear that the process of development has 
undergone its maximum of abbreviation. The embryo 
presents no distinct and independent Nauplius or Zowa 
stages, and, as in the crab, there is no Schizopod or 
Mysis stage. The abdominal appendages are developed 
very early, and the new born young, which resembles the 
Megalopa stage of the crab, differs only in a few points 
from the adult animal. 
Guided by comparative morphology, we are thus led 
to admit that the whole of the Arthropoda are connected 
by closer or more remote degrees of affinity with the 
crayfish. If we were to study the perch and the pond- 
snail with similar care, we should be led to analogous 
conclusions. For the perch is related by similar grada- 
tions, in the first place, with other fishes; then more 
remotely, with frogs and newts, reptiles, birds, and 
mammals; or, in other words, with the whole of the 
great division of the Vertebrata. The pond-snail, by 
like reasoning upon analogous data, is connected with 
the Mollusca, in all their innumerable kinds of slugs, 
shellfish, squids, and cuttlefish. And, in each case, the 
study of development takes us back to an egg as the 
primary condition of the animal, and to the process of 
yelk division, the formation of a blastoderm, and the con- 
version of that blastoderm into a more or less modified 
