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 r 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, where 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, which 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 
individuals unite into one mass, surround themselves with 
a case, in which they divide into several parts, each por¬ 
tion becoming a new individual. 
The sperm-cells differ from the egg in being very small, 
