42 INVEBTEBBATS MOBPHOLOOT. 



It has been pointed out that the Flagellate Volvox presents 

 a tendency towards a higher individuality, being somewhat 

 higher than a mere colony of cell-individuals and yet not 

 quite reaching the dignity of an organ-individual ; similarly 

 intermediate conditions between the other grades may occur. 

 In certain worms, for instance, considerable independence of 

 the constituent metameres exists, any one of them, when de- 

 tached, being capable of carrying on an independent exist- 

 ence, and of developing into an organism similar to that of 

 which it was originally a part. In the Earthworm the depend- 

 ence of the various segments or metameres upon one another 

 is greater than this, but in it, too, a certain amount of inde- 

 pendence is shown by the power it possesses of regenerating 

 lost metameres. In other cormi, as, for instance, in the 

 Lobster, the interdependence of the component metameres 

 proceeds still farther, and a differentiation of the various meta- 

 meres occurs, a process carried to its greatest extent in the 

 higher Vertebrates. A physiological division of labor among 

 the metameres develops, some of them losing, for instance, 

 their excretory organs, while in others these organs lose their 

 excretory functions and serve as ducts by which the repro- 

 ductive elements may pass to the exterior. The subordina- 

 tion of the metameres proceeds most rapidly and is most 

 complete at the anterior extremity of the organism, leading 

 to the formation of a head bearing highly developed sense- 

 organs and containing a complex nervous system, which rep- 

 resents originally distinct metamere nervous systems, now 

 fused and destitute of all independence. 



Sexual Reproduction in the Metazoa. — In cell-individuals it 

 has been seen that fission is the most frequent and simplest 

 mode of reproduction ; in the Metazoa this method and its 

 modification, budding, also occurs, but, as a rule, only in 

 forms of a low grade of individuality or in a transition stage 

 between a lower and a higher grade. In organ-individuals 

 it is of frequent occurrence, the imperfect separation of the 

 individuals so produced leading, in many cases, to the forma- 

 tion of colonies, and in cormi in which the integration of the 

 constituent metameres is but slight it also occurs. 



In the Protozoa cell-division naturally entails reproduc- 



