THE MORPHOLOGICAL COMPOSITION OF ANIMALS. Ill 



to think has gone on among plants. Minute aggregates of 

 those physiological units which compose living protoplasm, 

 exist as Protozoa : some of them incoherent, indefinite, and 

 almost homogenous ; and others of them more coherent, de- 

 finite, and heterogenous. By union of these nucleated parti- 

 cles of sarcode, are produced various indefinite aggregates of 

 the second order — Sponges, Thalassicollse, Foraminifers, &c. ; 

 in which the compound individuality is scarcely enough 

 marked to subordinate the primitive individualities. But in 

 other types, as the Hydra, the lives of the morphological 

 units are in a considerable degree, though not wholly, merged 

 in the life of the integrated whole they form. As the primary 

 aggregate when it passes a certain size undergoes fission or 

 gemmation ; so does the secondary aggregate. And as on 

 the lower stage so on the higher, we see cases in which the 

 gemmiparously-produced individuals part as soon as formed, 

 and other cases in which they continue united, though in great 

 measure independent. This massing of secondary aggregates 

 into tertiary aggregates, is variously carried on among the 

 Hydrozoa, the Actinozoa, and the Molluscoida. In most of the 

 types so produced, the component individualities are very 

 little subordinated to the- individuality of the mass they form 

 — there is only physical unity and not physiological unity ; 

 but in certain of the oceanic Hydrozoa, the individuals are so 

 far differentiated and combined as very much to mask them. 

 Forms showing us clearly the transition to well-developed 

 individuals of the third order, are not to be found. Never- 

 theless, in the great sub-kingdom Annulosa, there are traits 

 of structure, development, and mode of multiplication, which 

 go far to show that its members are such individuals of the 

 third order; and in the relations to external conditions 

 involved by the mode of union, we find an adequate cause for 

 that obscuration of the secondary individualities which we 

 must suppose has taken place. The two other great sub- 

 kingdoms MoUusca and Vertebrata, between the lower mem- 

 bers of which there are suggestive points of community, 



