350 



PRINCIPLES OF ANIMAL BIOLOGY 



differences that make it possible to recognize the group of animals by 

 the labyrinth alone. The embryos of animals also show homologies. 

 Every college course in embryology is a recognition of the existence of 

 types of development; for the laboratory studies, based on one or two 

 animals, are used to exemplify most of the classes in a phylum. The 

 homology of embryos is more spectacular when it is discovered in species 

 that are not alike in the adult. This situation is more likely to arise in 



Fig. 298. — Membranous labyrinths of inner ear of various vertebrates. Each consists 

 of a saccular portion from which three semiciicular canals arise. A, of a fish; B, of a frog; 

 C, of a reptile; D, of a bird. (^Modified from Retzius.) 



parasitic animals, since adult parasites are frequently very degenerate. 

 An excellent example is a parasite, Sacculina, found attached to the under- 

 side of the abdomen of common crabs (Fig. 299). Sacculina, in the adult 

 stage, is a rounded pulpy mass with practically no definite structure, 

 except a host of rootlike processes which extend throughout the crab's 

 body and absorb the body fluids. The embryo, however, is a three- 

 cornered little animal with jointed legs which clearly marks Sacculina as 

 one of the Crustacea. It is, in fact, one of the barnacles, a group in which 

 adult structure is usually quite complicated (Fig. 300). 



