velopment, the new individual is pushed out of the mother's body by con- 

 tractions of the uterus. The cord breaks off close to the abdomen; the scar 

 formed is called the navel. Later the placenta and the cord are also forced out; 

 this is the so-called "afterbirth". 



How Do Invertebrates Reproduce? 



Reproduction among Insects There seems to be a direct relation be- 

 tween the modes of reproduction in different classes of plants and animals and 

 the conditions under which the various species live. In the life of each in- 

 dividual organism the earliest stages of the egg's development are passed in 

 water. Only in the more complex forms are the latest stages passed on land 

 or in the air. But whereas spores appear, in general, to be adapted to endure 

 and sur\'i\'e drought, sperm and Qgg cells are quickly killed when away from 

 moisture. Some of the distinctive characteristics of land and air species may 

 be regarded as adaptations to the fertilization of tgg cells while they are still 

 in the body of the mother. 



Among insects, which of all classes of animals are most definitely adapted to 

 life in the air, the sperm cells of the male, suspended in a fluid, are passed 

 directly into a receptacle in the body of the female through a special duct, or 

 tube. From this receptacle the sperm cells pass, a few at a time, into another 

 space, in which the eggs are fertilized. A queen bee can retain a quantity of 

 li\'ing sperms for two or three years, or even much longer. She forces sperms 

 out of the receptacle from time to time as she produces new eggs. 



Even in insects that normally lay their eggs in the water, such as mosquitoes, 

 fertilization takes place within the body of the mother. There is a wide 

 range, however, between species that leave the eggs as soon as they are laid 

 and those others (like the wasps, bees, and ants) that build elaborate nests for 

 the eggs and young, store away food, and actually nurse or protect the young. 



Aquatic Invertebrates Among invertebrate animals li\^ing in the water, 

 such as sponges, corals, starfish, clams, and some crustaceans, eggs and sperms 

 are discharged into the water. The eggs contain relatively large amounts of 

 food material and sink to the bottom. The swimming sperm cells swarm 

 about the eggs. Fertilization takes place when one sperm penetrates the 

 protoplasm of an egg. 



When the nuclei of the germ cells have united, the fertilized egg cell forms 

 a denser membrane. Other sperms cannot then enter. In some species of w^ater 

 animals, segmentation, or cell-division, starts immediately; in others there 

 are varying intervals. In most species the fertilized ^ggs, like the gametes, 

 are abandoned by the parents. In some invertebrates, however, the eggs re- 

 ceive a degree of mechanical protection. In lobsters and crayfish, for example, 

 the eggs are fastened to the abdominal legs of the mother by a sticky substance, 



381 



