18 INVERTEBRATE ZOOLOGY 



germ cells together and thus insure fertilization. Under such 

 conditions, the larva or young of Metazoa passes through its 

 embryological stages until it is capable of independent mainte- 

 nance. Among many Metazoa, this stage is reached with the 

 gastrula, for at this time the larva contains organs differentiated 

 sufficiently to enable it to ingest and utilize food from the outside 

 world. For its metabolism prior to this time, the embryo has 

 been dependent upon the food substances stored within the egg. 

 Correlated with increase in quantity of stored food material 

 within the egg, the young of many species undergo complete 

 development upon the materials thus furnished, as mentioned 

 above, and never lead an independent larval existence. 



Frequently, the eggs are fertilized while still within the body of 

 the female. This may involve a copulation whereby sperm cells 

 are deposited within the body of the female by the male through 

 some sort of intromittent organ, or, in some instances, as in 

 the rotifers and leeches, sperm cells are united in groups called 

 spermatophores which penetrate the body wall of the female. 

 Parts of the body of the female may be modified as a seminal 

 receptacle for receiving the sperm cells from the male, though 

 fertilization of the eggs may be deferred for some time, even 

 several years, after copulation has taken place. In hermaphro- 

 ditic forms, there are several distinct methods of fertilization. 

 Reciprocal copulation (Fig. 78), whereby sperm cells from one 

 hermaphroditic individual fertilize the eggs of the other, occurs in 

 many instances. Self-fertilization is also encountered in some 

 hermaphroditic organisms. 



Birth Habits. — When eggs are discharged before cleavage has 

 begun, the species is said to be oviparous. Frequently, the egg 

 is retained within the body of the female until the young is fully 

 formed and the larva emerges from the egg membranes before it 

 leaves the body of the parent. This condition is designated as 

 viviparous. Any condition intermediate between these two 

 extremes is termed ovoviviparous. In this condition, the egg has 

 at least started to divide before it leaves the body of the parent. 

 In popular, and sometimes in scientific, usage the term oviparous 

 is applied to any condition in which the female brings forth eggs, 

 regardless of the stage of development of the contained ovum or 

 embryo. The term ovoviviparous is in this instance restricted 

 to that condition in which the embryo is liberated from the egg 

 membranes just before it leaves the maternal body. 



