196 



ELEMENTS OF BIOLOGY 



a more or less complicated set of secondary sex organs or charac- 

 ters may occur; for example, the spurs, head furnishings, and colora- 

 tion of male birds, the thumb pad of the male frog, the body form 

 and mammary glands of female mammals, are secondary sex char- 

 acters. There are wide variations and diflFerences in both primary 

 and secondary sex characters; to review them in detail is beyond 

 the purposes of this discussion. It is not possible to establish a series 



TESTIS [MALE) 



SPERM DUCT 

 {MALE) 



OVARY (FEMALE) 



OVIDUCT 

 (FEMALE) 



SEMINAL 

 RECEPTACLES 

 {FEMALE) 



SEMINAL 



VESICLE 



{MALE) 



Fig. 134. — Diagram of the reproductive organs of an earthworm. 



of increasing complexity nor to generalize concerning the principles 

 of organization of reproductive systems. For example, in such dis- 

 similar forms as worms, arthropods, birds, reptiles, and the lower 

 mammals, the walls of the egg ducts or associated glands secrete 

 and deposit shells about the eggs as they pass. In invertebrates the 

 shell is composed of chitin; in vertebrates it is largely calcium car- 

 bonate. On the other hand, within a phylum the arrangement of 

 the sex organs may differ widely between classes. For example, 

 within the phylum Arthropoda, in members of the class Crustacea, 

 lobsters, crayfish, et cetera, the gonads are largely located in the 



