82 MORPHOLOGICAL DEVELOPMENT. 



component units remain quite distinct. Being, as aggro- 

 gates of the first order, much more definitely organized, 

 their union into aggregates of the second order does not de 

 stroy their original individualities, Among the f^orticelhs, 

 of which two kinds are delineated in Figs. 144 and 145, there 

 are various illustrations of this : the members of the com 

 munity being sometimes appended to a single stem ; some 

 times attached by long separate stems to a common base ; and 

 sometimes massed together. 



Thus far, these aggregates of the second order exhibit but 

 indefinite individualities. The integration is physical ; but 

 not physiological. Though, in the Thalassicollw, there is a 

 shape that has some symmetry ; and though, in the Fora- 

 mini/era, the formation of successive chambers proceeds in such 

 methodic ways, as to produce quite-regular and tolerably- spe 

 cific shells ; yet no more in these than in the Sponges or the 

 compound Vorticellce, do we find such co-ordination as gives 

 the whole a life predominating over the lives of its parts. 

 We have not yet reached an aggregate of the second order, 

 so individuated as to be capable of serving as a unit in still 



higher combinations. But in 

 the class Goelenterata, this ad 

 vance is displayed. The com 

 mon Hydra, habitually taken as 

 the type of the lowest division 

 of this class, has specialized 

 parts performing mutually-subservient functions ; and thus 

 exhibiting a total life distinct from the lives of the units. 

 Fig. 146 represents one of these creatures in its contracted 

 state and in its expanded state ; while Fig. 147 is a 

 rude diagram from memory showing the wall of this 

 creature s sack-like body as seen in section under the 

 microscope : a and b being the outer and inner cellular 

 layers ; while in the central space between them, is 

 that nucleated substance, or sarcode, or protoplasm, 

 in which the cells originate. But this lowly- organized 



