134 How Animals Changed 



connecting specialized with primitive conditions, or individuals, 

 races, or species may be sharply segregated from others. Land crus- 

 taceans, which came originally from the sea, either produce marine 

 free-swimming larvae from their eggs (Coenobita, Birgus, Uca, 

 Cardisoma, Gecarcinus) or they do not (crayfishes; river crabs, 

 Potamon) . There are no intergrades. The littoral isopod Ligia 

 baudiniana (Milne-Edwards) commonly avoids sea water and, when 

 wetting its gills, carefully keeps its body from contact with it, but 

 does not free its young from its brood-pouch unless submerged. 

 The British littoral snails of the genus Littorina furnish a nice 

 series of variations in breeding habits: L. littorea (L.), lowest on 

 the beach, produces eggs which hatch out early veliger larvae; L. 

 obtusata (L.) in the mid-tidal zone lays eggs which hatch out sec- 

 ond-stage veligers; L. saxatilis (Oliv.) and L. neritoides L. are 

 viviparous and thus best adapted to the dryer conditions near high- 

 tide mark (Colgan, 1910; Flattely & Walton, 1922). 



Some animals in their life cycles reproduce events which repre- 

 sent critical transformations that their ancestors struggled to attain 

 in the past. A tadpole when it metamorphoses into a frog changes 

 and improves its powers of regulating osmotic pressures within its 

 body, and the acquirement of this new ability is simultaneous with 

 the cessation of the use of the gills (Adolph, 1927b). A few fishes 

 "undergo a certain metamorphosis into partly terrestrial animals, 

 and Harms found that this metamorphosis was influenced by the 

 thyroid hormone, as in the case of Amphibia" (Noble, 1931). The 

 land hermit crab Birgus latro Leach, when young, inserts its twisted 

 abdomen in a snail's shell, like other hermit crabs. As it grows 

 larger, it finally is unable to find a shell big enough for its abdo- 

 men, which then becomes bilaterally symmetrical and bears hard 

 chitinous plates on its dorsum (Harms, 1932) . 



The development of an animal requires a certain environment, 

 which in many cases is the sea or is a sea-like liquid enclosed within 

 an egg shell. Energy is also necessary, and, as a rule, much of this 



