164 GENERAL PRINCIPLES OF ZOOLOGY. 



;i cutting off of a part of the entodermal ei3ithelmm, epithelial 

 mesoderm, mesothclivm; or by the migration of separate cells to 

 form a gelatinous tissue {mesenchyme). 



25. Many animals dejiosit their eggs before or shortly after 

 fertilization {oviparous) ; others lay eggs which have already been 

 fertilized in tlie maternal body, and at the time of laying have 

 passed through some of the stages of development (ovo-viviparoiis). 

 A third series of animals give birth to living young {vivipai'ous). 



20. The development of an animal is either direct or indirect 

 (metamorphosis). 



27. Indirect development or metamorphosis is where the young 

 animal, as it comes from the egg, differs from the sexually mature 

 animal in two points : 



(«) by the lack of certain organs which occur in the 



sexually mature animals; 

 (J) by the appearance of organs, larval organs, which are 

 lacking in the sexually mature animals. 



III. RELATION OF ANIMALS TO ONE ANOTHER. 



General Relations. — Just as between the organs of one and the 

 same animal there exists a regular relation which is termed corre- 

 lation of parts, so also the different individuals of the animal 

 l^opulation stand in manifold and intimate reciprocal relations to 

 one another. Darwin has shown, in a great number of instances, 

 how the conditions of existence of many animal species are altered, 

 if other forms ajipear or disappear, or undergo an extraordinaiy 

 reduction or increase in number of individuals. Such reciprocal 

 effects are usually of a more special nature and can be understood 

 only by individual study; a few conditions are of wide occurrence 

 and are hence suitable for a general consideration ; to such belong 

 colony and society formation, parasitism, and symbiosis. 

 I. Relations hetwben" Individuals of the Same Species. 



Colony Formation. — Colony and society formation are condi- 

 tions which exist between individuals of the same species. An 

 animal roloni/ is a uiiion of numerous individual animals bv an 

 organic bodily connexion; the latter may arise in two wavs: first, 

 by animals, originally separate, approaching one another and 

 partially fusing together; secoiidly, by individuals, formed by 

 division and budding, remaining united with one another instead 

 of separating. The first is extremely rare> and in the animal 

 kingdom plays no role whatever. 



