90 MORPHOLOGICAL DEVELOPMENT. 



not destroy their original individualities. Among the Vorti- 

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

 145, there are various illustrations of this: the members of 

 the community being sometimes appended to a single stem; 

 sometimes 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 Polycytlwria, 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 toler- 

 ably-specific 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 Ccelenterata, this 

 advance is displayed. The common 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 diagram 



showing the wall of this crea- 

 ture's sac-like body as seen in 

 section under the microscope: 

 a and b being the outer and 

 inner cellular layers; while be- 

 tween them is the " mesogloea " 

 or " structureless lamella," the 

 supporting or skeletal layer. But this lowly-organized 

 tissue of the Hydra, illustrates a phase of integration 

 in which the lives of the minor aggregates are only par- 

 tially-subordinated to the life of the major aggregate 

 formed by them. For a Hydra's substance is separable 



