THE FROG AS A REPRESENTATIVE VERTEBRATE 23 



immature worms of this type are often found in the intestine and 

 the body cavity. In the large intestine of almost every specimen 

 examined, there will be found, in addition to many bacteria, sev- 

 eral species of single-celled parasites belonging to the group of 

 animals known as Protozoa. The Hst might be still further 

 extended. It is not uncommon to find a dozen or more different 

 species of parasites in a single frog taken at random. Since most 

 parasites are specialists, to the extent that they infest but a single 

 species of host or a few closely related species, and since most well- 



FiG. 11. — Development of the frog. 



A, eggs. B, C, D, and E, cleavage, gastrula, and neural fold stages. F, newly hatched 

 tadpoles. G and H, later tadpole stages. I, J, and K, metamorphosis to juvenile frog. 



known animals have many parasites, it is not impossible that the 

 total number of parasitic species of animals exceeds those that are 

 free-Uving. Parasitism is, therefore, of widespread occurrence 

 and presents many interesting biological problems. There are 

 few, if any, true cases of parasitic vertebrates, probably for the rea- 

 son that most vertebrates are animals of some size, while parasites 

 are, of necessity, smaller animals. 



Seasonal Changes and Life Cycle. — Following its breeding 

 season in early spring, the leopard frog leads an active Hfe, feeding 

 voraciously to restore the loss entailed by its "winter sleep" and 



