Reproduction 



133 



salt in fresh water and salts must be provided in the egg (Baldwin, 

 1937) . (6) Adoption of peculiar breeding habits to avoid interspe- 

 cific competition. The grunion deposits eggs in sand above the 

 water, and other smelts leave their eggs in the sea. The species of 



Fig. 22. A male fiddler crab waving at a female. On land sight and 

 hearing are emphasized more than taste and smell in courtship. 



shrimps in Chesapeake Bay breed at different seasons (Cowles, 

 1908) . "The terrestrial stage in the life of Amhystoma opacum is 

 an adaptation permitting the species to compete successfully in the 

 same region with other species of Amhystoma' (Noble & Brady, 

 1933) . Of course no single quality of -those enumerated is abso- 

 lutely characteristic of land or fresh water, but they indicate trends. 

 There are marine elasmobranchs which nourish their young with 

 secretions from the mother's uterus (Alcock, 1902) . 



In life cycles animals may show nicely graded series of stages 



