406 COMPARATIVE ZOOLOGY 



As to form, eggs are oval or elliptical, as in birds and 

 crocodiles ; spherical, as in turtles and wasps ; cylindri- 

 cal, as in bees and flies ; or shaped like a handbarrow, 

 with tendrils on the corners, as in the shark. The eggs 

 of some very low forms are sculptured or covered with 

 hairs or prickles. 



The number of eggs varies greatly in different ani- 

 mals, as it is in proportion to the risks during develop- 

 ment Thus, the eggs of aquatic tribes, being unprotected 

 by the parent, and being largely consumed by many 



FIG. 360. Egg of a Shark (the external gills of the embryo are not represented). 



animals, are numerous to prevent extinction. The spawn 

 of a single cod contains millions of eggs ; that of the 

 oyster, 6,000,000. A queen bee, during the five years 

 of her existence, lays about a million eggs. 



Eggs are laid one by one, as by birds ; or in clusters, 

 as by frogs, fishes, and most invertebrates. The spawn 

 of the sea snails consists of vast numbers of eggs 

 adhering together in masses, or in sacs, forming long 

 strings. 



As a rule, the higher the rank, the more care animals 

 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 (Unid) carries them within its gills, and 



