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 Polycytharia, 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 Ccslenterata,, 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 



