196 



COMPARATIVE ZOOLOGY. 



take of their eggs and their young, and the higher the 

 temperature needed for egg-development. In the major- 

 ity of cases, eggs are left to themselves. The fresh-water 

 Mussel (Unio) carries them within its gills, and the Lob- 

 ster under its tail. The eggs of many Spiders are envel- 

 oped in a silken cocoon, which the mother guards w T ith 

 jealous care. Insects, as Flies and Moths, deposit their 

 eggs where the larva, as soon as born, can procure its own 

 food. Most Fishes allow their spawn, or roe, to float in 

 the water ; but a few build a kind of flat nest in the sand 

 or mud, hovering over the eggs until they are hatched ; 

 while the Acara of the Amazons carries them in its 

 mouth. The Amphibians, generally, envelop their eggs 

 in a gelatinous mass, which they leave to the elements ; 

 but the female of the Surinam Toad carries hers on her 

 back, w T here they are placed by the male. The great Am- 

 azon Turtles lay their eggs in holes two feet deep, in the 

 sand ; while the Alligators simply cover theirs with a few 

 leaves and sticks. Nearly all Birds build nests, those of 

 the Perchers being most elaborate, as their chicks are de- 

 pendent for a time on the parent. 109 The young of Mar- 

 supials, as the Kangaroo, w T hich are born in an extremely 

 immature state, are nourished in a pouch outside of the 

 body. But the embryo of all other Mammals is devel- 

 oped within the parent to a more perfect condition, by 

 means of a special organ, the placenta. It is a general 

 law, that animals receiving in the embryo state the longest 

 and most constant parental care ultimately attain the high- 

 est grade of development. 



The Protozoa, which have no true eggs, have a sort of 

 reproduction called conjugation. In this process two 

 Amoebae unite into one mass, surround themselves with a 

 case, in which they divide into several parts, each portion 

 becoming a new Amoeba. 



The sperm-cells differ from the egg in being very small, 



