352 



THE GEOLOGIST. 



We proceed now to detail tlie chief cliaracters of eacli of the 

 above great groups, beginning with the lowest, and the type-plans 

 - on which they are constructed. To say what is the lowest form of 

 animal-life is indeed difficult, as it is also of the vegetable. "We 

 have certain moving and apparently living cells and frustules, such 

 as the monads, diatoms, and other organisms, commonly termed 

 (from their usual presence in vegetable infusions and stagnant 

 water) Infusoria ; but the discussion is still strong as to their proper 

 position, although the stronger evidence is at present on the side of 

 those who group them with the vegetable kingdom. At all events 

 the minutest and simplest forms of organized objects are simple 

 cells ; and as all animals and vegetables, whatever their rank, are 

 built up of an organic structure composed of cells, it is at least 

 dif&cult to determine either the identity with each other of such 

 primitive cells ; the commingling in them of the rudimentary 

 stages of both kingdoms ; or from their smallness, delicateness, and 

 similarity of chemical composition and their structural resemblances, 

 to point out the essential distinctions. 



These primitive cell-forms, too, possessed of no solid parts, and 

 liable to almost instant decay after death, enter not into the domains 

 of palaeontology ; and it is only in the case of the diatoms and 

 siliceous or calcareous loricated (shelled) forms that we find any 

 traces in a fossil state. These, however, are found in such abund- 

 ance in some of the later Tertiary deposits as to form whole beds, 

 which are, as in the case of the Tripoli and the Berg-mehl, used for 

 industrial or domestic purposes. But as these belong, according to 

 most authorities, to the vegetable tribes, we shall notice them more 

 fully under that head and when we come to treat in the progTess of 

 our work of the Tertiary rocks. 



The first class then which we notice of the Protozoa are the 

 Rhizopoda or root-foot animals. The Protozoa are all more or less 

 cjlohular or amorphous ; for even the sponges or Porifera have 

 generally a spherical shape, although in some genera of this 

 higher gToup the true globular form is indented into funnel- 

 shaped cavities or elongated into tubular stems. In the lower 

 group recognized in the fossil-state by the innumerable shells of 

 Foraminifera, abundant in many rocks, but best known in the 



