48 



NATURAL HISTORY 



Huijh Spiricer 



A spider with its egg cocoon. 



until large enough to care for themselves. The male of the so-called 



midwife toad (Alytes) carries the eggs entangled around the legs. 



The male Surinam toad places the eggs on the back of the female, 



where each sinks into a tiny pouch as it develops. 



Animals that lay eggs which hatch outside of the mother's body 



are said to be oviparous. A modified form of this procedure is seen 



in some nematodes, arthropods, 

 fish, amphibia, and reptiles. Here 

 the eggs remain in the oviduct or 

 uterus of the mother until they 

 are almost ready to hatch, the 

 body of the mother acting as an 

 incubator. Such forms are said 

 to be ovoviparous. Most of the 

 mammals which retain the eggs in 

 the body until the young are born 

 are said to be viviparous. Here 

 the young are held as embryos 

 within the body of the mother 



and nourished by means of an organ called the placenta. The young 



of mammals are suckled at the breasts of the mother until they 



are able to eat solid food. 



Relations of Mutual Aid 



A certain amount of protection is afforded plants from their habit 

 of living in communities. Examples are the aggregations of cacti in 

 our western deserts or the acacia and "thorn bush" communities of 

 Australia. The animal world, too, shows many examples of protec- 

 tion among gregarious forms. The schooling of fishes not only is a 

 defense for the group from larger fish, but it also enables small fish, 

 working concertedly, to prey on organisms much larger than them- 

 selves. The driver ants in Africa, traveling in great swarms, often 

 overcome and devour animals hundreds of times larger than them- 

 selves. Wolves hunt in packs, several of them rushing together to 

 bring down their larger prey. Deer and other herbivorous animals 

 move in herds for mutual protection. 



Another relation of mutual aid results from the development of 

 division of labor among certain animals. Although social division 

 of labor is well seen in the human species, there are many examples 

 in the insect world, particularly among the social bees and wasps, 



