THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 79 



forms like that of the Amoeba ; and from this young condi- 

 tion in which they are undifferentiated, they pass into that 

 adult condition in which they haye limiting membranes. If 

 this development of the individual Qregarina typifies the 

 mode of evolution of the species, it yields further support to 

 iha belief, that homogeneous fragments of sarcode existed 

 earlier than any of the structures which are properly called 

 cells. Among aggregates of the first order, there 



are some much more highly developed. These are the Infu- 

 soria; constituting the most numerous of the Protozoa, in 

 species as in individuals. Figs. 137, 138, and 139, are ex- 

 amples. In them we find, along with greater definiteness, 

 a considerable heterogeneity. The sarcode of which the body 

 consists, has an indurated outer layer, bearing cilia and some- 

 times spines ; there is an opening serving as mouth, a per- 

 manent oesophagus, and a cavity or cavities, temporarily 

 formed in the interior of the sarcode, to serve as one or more 

 stomachs ; and there is a comparatively specific arrangement 

 of these and various minor parts. 



Thus in the animal kingdom, as in the vegetal kingdom, 

 there exists a class of minute forms having this peculiarity, 

 that no one of them is separable into a number of visible com- 

 ponents homologous with one another — no one of them can 

 be resolved into minor individualities. Its proximate units 

 are those physiological units of which we conclude every or- 

 ganism consists. The aggregate is an aggregate of the first- 

 order. 



§ 201. Among plants are found types indicating a transi- 

 tion from aggregates of the first order to aggregates of the 

 second order ; and among animals we find analogous types. 

 But the stages of progressing integration are not here so dis- 

 tinct. The reason probably is, that the simplest animals, 

 having individualities much less marked than those of the 

 simplest plants, do not afford us the same facilities for ob- 

 servation. In proportion as the limits of the minor indi- 



