124 MORPHOLOGICAL DEVELOPMENT. 



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

 almost homogeneous, and others of them more coherent, de- 

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

 cles of sarcode, are produced various indefinite aggregates of 

 the second order Sponges, Polycytharia, Foraminifers, &c. ; 

 in which the compound individuality is scarcely enough 

 marked to subordinate the primitive individualities. But in 

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

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

 in the life of the integrated body 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 aggre- 

 gates into tertiary aggregates, is variously carried on among 

 the Hydrozoa, the Actinozoa, the Polyzoa, and the Tunicata. 

 In most of the types so produced, the component individu- 

 alities are very little subordinated to the individuality of the 

 composite mass there is only physical unity and not physio- 

 logical 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 transi- 

 tion to well-developed individuals of the third order, are not 

 to be found. Nevertheless, in the great sub-kingdom Annu- 

 losa, 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 indi- 

 vidualities which we must suppose has taken place. The 

 two other great sub-divisions, Mollusca and Vertebrata, 

 between the lower members of which there are suggestive 

 points of community, present us only with aggregates of the 

 second order, that have in many cases become very large and 



