THE HISTORY OF ANIMAL LIFE 413 



ing water; the formation in mammalian embryos of the same basic 

 structures, pharyngeal pouches, from which the gills arise in the 

 fishes, and so on. The list is already extensive and is being added to 

 from time to time as the significance of structures in the embryo 

 becomes apparent. These embryonic vestiges, disappearing as they 

 do before birth, admit of only one rational explanation, namely, 

 that their significance is historical, that they constitute remnants of 

 ancient ancestral inheritance. 



Evidence from Parasitism. By comparing the anatomy and 

 the development of parasitic animals with free-living forms that 

 are similar, strong evidence of descent with change comes to light. 

 But the changes have been retrogressive rather than progressive, for 

 parasites of all sorts show either embryonic or adult vestiges of 

 structures and organ systems that are well developed and func- 

 tional in closely related free-living animals. The trend of degenera- 

 tive changes that have accompanied the transition from free-living 

 to parasitic life is illustrated by the three classes of Platyhelminthes, 

 Turbellaria (free-living), Trematoda (parasitic), and Cestoda (para- 

 sitic). The Trematoda (example, the liver fluke), while resembling 

 the Turbellaria (example, Planaria) in many ways, lack eyes and 

 show other reductions in the nervous system. On the other hand, 

 the reproductive organs of the Trematoda are the more conspicu- 

 ous and active. The Cestoda (example, the tapeworm) not only 

 lack eyes but also an alimentary tract. Moreover, the nervous sys- 

 tem of the cestode is even less well developed than that of the 

 Trematoda. On the other hand, the gonads of cestodes are still 

 more conspicuous; mature proglottids of the tapeworm, for in- 

 stance, consist almost entirely of ovaries and testes and are packed 

 with fertilized eggs. In a previous chapter (p. 365) attention was 

 directed to an extreme case of degeneracy in a parasite, Xenoccrlotiia 

 brumpti, a small arthropod that parasitizes a marine annelid. Poly- 

 cirrus. The adult form of this parasite, if it may be called adult, 

 consists of ovaries, testes, and genital ducts. The existence of an 



