MULT1CELLULAR ORGANIZATION: PHYLOGENETIC 



perform or help perform a definite function. As an example of an animal 

 belonging to this second division of the third order of multicellular 

 organization, we may cite Hydra. 



Multicellular organization in the narrower sense of the term ends here ; 

 but this does not cover the scope of multicellular organization. Just as 

 cells are assembled to form a colony in which certain of them are differen- 

 tiated and segregated to form tissues, so tissues have been differentiated 

 and segregated to perform definite functions or sets of functions, giving 

 rise to organs. An organ, then, is a group of differentiated tissues per- 

 forming some particular function. 



Multicellular organisms, therefore, range from groups composed of 

 an indefinite number of cells scarcely, if at all, related to each other, to 

 individuals composed of closely interrelated and mutually dependent 

 organs. It is also held that the phylogenetic series was equally as exten- 

 sive. This is rendered all the more likely by the evidence gathered from 

 the study of the ontogenetic series. 



An Example of a Colony of the First Order: Carchesium. The indi- 

 vidual cells in a colony of Carchesium are attached to a branched system 

 of stalks which they themselves have elabo- 

 rated. There is, in this system of stalks, 

 a set of radiating branches that arise from 

 a common point of attachment. From these 

 radial branches short lateral branches are 

 given off at more regular intervals. At the 

 end of each radial and each lateral branch 

 a single cell is borne (Fig. 13). Each ani- 

 mal is a bell-shaped cell measuring, when 

 extended, about fifty microns in diameter 

 and seventy-five microns in length. The 

 cytoplasm is differentiated into an ectoplasm 

 and an endo plasm. Within the endoplasm 

 there is a constant cyclosis. This endoplasm 

 contains a sausage-shaped macronucleus 

 and a single rounded micronucleus. These 

 nuclei are not carried with the cyclosis, but 

 have a stationary position in the cell. Open- 

 ing out from the ectoplasm is a single con- 

 tractile vacuole. At the distal end there is 

 a ciliated zone which makes about one and a half spirals as it winds 

 about the body to enter the "gullet." At the base of the gullet the 

 food passes through the mouth into the endoplasm to form a food 

 vacuole. The stalk of each animal is provided with a contractile fiber 

 (Fig. 13, /.) 



f! 



FIG. 13. A colony of individuals 

 of Carchesium attached by a com- 

 mon branching stalk ; /, contract- 

 ile stalk of one individual. (REMY 

 after PERKIER.) 



