150 GENERAL ZOOLOGY 



great systems which are chiefly as follows: digestive, circu- 

 latory, respiratory, excretory, muscular, skeletal, nervous, 

 reproductive. 



REPRODUCTION IN METAZOA 



It has been said that all Metazoa start as single cells, and 

 in general this statement is true, but there are many excep- 

 tions in particular cases. In fact, it may be said that there 

 are two methods of reproduction sexual and asexual. 

 The former always involves the union of an ovum and a 

 spermatozoon to produce a new and young individual. 

 Asexual means without sex, and there are various ways in 

 which new individuals may be formed without the usual 

 fusion of germ cells. Some sponges may be cut up in pieces 

 and propagated as slips, like plants; others give off groups of 

 cells as buds, and these grow into new individuals, still 

 others have the bulk of the body decay at certain seasons, 

 but at such times groups of cells in the interior form a re- 

 sistant coating, live until they can have opportunity to grow 

 again, and then form a complete new sponge. Various 

 polyps and worms are able to produce new individuals by 

 budding off portions of the body or by breaking it in two. 

 In addition to usual asexual methods of reproduction, 

 there are among metazoans some modifications of sexual 

 processes. Many-celled animals show two conditions in 

 regard to sex they may be hermaphroditic or dioecious. 

 In the former case an animal has both male and female 

 organs in its body, and in rare instances is able to fertilize 

 itself. It can thus sometimes produce offspring without 

 the presence of another individual. There is another 

 modification of sexual reproduction shown in partheno- 

 genesis. An ovum will usually not develop unless it has 

 united with a sperm cell but there are a few animals in 

 which such union may not be necessary, and ova therefore 

 develop without fertilization. However, in most cases, 

 perhaps in all, parthenogenetic reproduction cannot go on 



