246 CLASSIFICATION AND ORGANIZATION OF ANIMALS 



the absence of a true enteron or gut cavity; the enterocceloiis- 

 diplobJastic type in which there is an enteron with a single opening 

 that functions as both mouth and anus, and in which there are 

 only two principal cell layers, the ectoderm and endoderm, as in ihe 

 Phylum Coelenterata (Fig. 121, p. 250); the enteroccelous-triplohlastic 

 type, as with the Platyhelminthes (Fig. 118), in which the enteron 

 has only one opening, although there is a middle cell layer, the 

 mesoderm, between ectoderm and endoderm; the coeloiuocoelous- 

 triploblastic type in which there is, in addition to the enteron, 

 a coelomic cavity, as in Annulata, Vertebrata, Mollusca, and 

 Echinodermata (Fig. 118); and the hmnoccclous-triploblastic type, 

 in which a coelome is not present although there are extensive 

 cavities, the hsemocoeles as in the Arthropoda {cf. p. 331). Coelo- 

 mocoelous and haemocoelous types are necessarily triploblastic, 

 since coelome and hsemocoele are cavities within the mesoderm 

 cells which lie between ectoderm and endoderm. 



Homology and Analogy. — Such classifications as the foregoing 

 necessitate consideration of the resemblance in essential structure, 

 which is termed homology; and the resemblance in function, termed 

 analogy. The fore limbs of amphibians, reptiles, birds, and mam- 

 mals are " homologous " whatever their functions, because all 

 are constructed upon the same plan (cf. Fig. 18, p. 39). They 

 may or may not be " analogous " structures since fore limbs may 

 have different functions, like the wings of birds, the flippers of 

 whales, the front legs of horses, and the arms of man {cf. Figs. 263 

 and 264, p. 500). The wings of a bird and those of an insect are 

 analogous since they have the same function, but not homologous 

 since they differ completely in structure. Again, the skeletons of 

 insects and vertebrates (Fig. 22, p. 42) are analogous but not homol- 

 ogous. Since the classification commonly used in biological 

 science is based upon structural resemblance, it is constantly 

 necessary, in classifying animals, to distinguish between what is 

 homologous and what is analogous. These principles are involved 

 in evolutionary problems, since structural resemblance is supposed 

 to indicate evolutionary relationship. For this reason classifica- 

 tion is commonly cited as one of the lines of evidence for organic 

 evolution (cf. p. 506). 



