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"orticeUcs t 

 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 Thalassicollce, there is a 

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

 minifera, 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 Caelenterata, 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 

 nide diagram from memory showing the wall of this 

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

 microscope : a and 6 being the outer and inner cellular 

 layers ; while in the central space between them, ia 

 that nucleated substance, or sarcode, or protoplasm, 

 in which the cells originate. But this lowly-organized 



