284 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



limbs, complete evidence of wliicli 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 mdependent Nauplius or Zoa^a 

 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 tliat 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 Vertehrata. 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 



