386 Prof. E. Forbes on the Morphology of the 



members of which are fixed, though serving different purposes in 

 the state. It is as truly a commonwealth as is the assemblage of 

 bees in their hive or of termites in their hill. In such common- 

 wealths we also see a division of physiological offices. Such com- 

 monwealths are to be found for the most part among beings in- 

 cluded in the articulate sphere of the animal kingdom; that sphere 

 which is itself representative of the vegetable kingdom, and obe- 

 dient to the same great general laws. 



Now as there are composite animals as well as plants, it be- 

 comes a curious and important inquiry to investigate the analo- 

 gies of their parts and functions, and to see how far our certain 

 knowledge of the plant will enable us to throw light on the na- 

 ture and regulating laws of the composite animal, at present very 

 obscurely understood. 



The present communication is intended to show, that in one 

 tribe at least of composite animals, in the Sertularian Polypes, the 

 arrangement and offices of individuals and of the parts of the ani- 

 mal entirely depend on the same laws which determine the ar- 

 rangement and offices of the parts of the composite plant. 



The Sertularian Polype is a branched and horny plant-like 

 polypidom, the axis of which is filled with living pith and the 

 branches studded with little cups or cells in which are seen the 

 fleshy polypes, each a stomach with arms around its mouth for 

 the seizing of its food. Each of these polypes is an individual 

 distinct in itself and acting for itself, yet, besides that individual 

 life, sharing in the common existence of the whole and obeying 

 in reference to its brethren the laws which determine the cha- 

 racters of the species — the constant form and arrangement of the 

 parts of the whole. If the axis should perish all the polypes 

 must perish, but one or several polypes may perish without affect- 

 ing the others or the life of the axis. 



Now all such polypes are true nutritive individuals, devoted 

 to the service of the composite individual or zoophyte of which 

 the polypidom is as it were the bark. The zoophyte begins as a 

 single individual, as the plant begins as a single phyton : polype 

 after polype is built up and shares in the common interest with 

 that first individual, as leaf after leaf is formed to serve in the 

 same commonwealth with the first phyton. The normal type of 

 the zoophyte is a simple stomach, that of the plant is a simple gill. 



At certain periods in the life of the zoophyte there appear pro- 

 jecting from the axis or springing from its branches variously 

 formed bodies, usually very dissimilar from the other parts of the 

 whole, in which the ova are after a time formed. These have been 

 called " vesicles," and many opinions have been entertained re- 

 specting their nature and origin. 



By most naturalists they have been styled evolutions from the 



