200 COMPARATIVE ANATOMY. 



family; just as a Tadpole might be placed by a person 

 who merely regarded its form, and was ignorant of its 

 history,in a widely-different family from the pai'ent Frog. 

 When first hatched, the young of the Crab presents the 

 singularly grotesque form represented in our figure. It 

 has a helmet-shaped head, terminating behind in a long 

 horn, and furnished in front with a pair of huge sessile 

 eyes, and it moves through the water by means of its 

 long swimming tail. After the first change of skin the 

 body assumes something like its permanent shape ; the 

 eyes become stalked ; the claws are developed, and the 

 legs resemble those of the Crab ; but the change is still 

 incomplete, for the tail is still long and furnished with 

 false feet, like that of a Lobster. The swimming habit 

 has not yet been laid aside. At the next stage, while 

 the little creature is still about the eighth of an inch in 

 diameter, the crab-form is completed, the abdomen fold- 

 ing in under the carapace. All the subsequent changes 

 are merely changes of coat, consequent on the growth of 

 the now complete animal. In these several metamor- 

 phoses we see portrayed, in succession, the peculiarities 

 of three different types, one rising above the other in 

 structure. In the first stage the Crab resembles one of 

 the least perfect Crustacea, such as the Water-flea ; in the 

 second it assumes the aspect of the Lobster ; and finally 

 puts on the form of the most perfect animals of the 

 class. Thus it is that Nature advances step by step, 

 gradually bringing out through successive stages of 

 being new organs and new faculties, and leaving as she 

 moves along, at every step, some animals that rise no 

 higher, as if to serve for land-marks of her doings 



