136 COMPARATIVE ANATOMY 



by the possession of pseudopodia, are classed together as 

 Rhizopoda. Amceba is one of the most simple members of 

 the group, and may be taken as a type of it, but it must not 

 be supposed that all Rhizopoda have so simple a structure. 

 Some of the nearest allies of Amoeba, having, like it, blunt 

 lobose pseudopodia, have the power of forming shells of 

 characteristic form for the protection of their bodies. Thus 

 Difllugia, a common fresh-water Rhizopod, protects itself by a 

 case formed of particles of sand glued together by a secretion 

 of the protoplasm. Arcella, another common fresh-water 

 form, has a watch-glass-shaped shell formed of a chitinoid 

 substance. The concavity of the watch-glass is covered in 

 by a plate with a central hole through which the pseudopodia 

 are protruded. Not far removed from these, and differing 

 from them chiefly in the fact that their pseudopodia are long 

 and thread-like, frequently branched at their extremities, and 

 anastomosing so as to form a protoplasmic network outside 

 the cell-body, are Euglypha, a fresh-water Rhizopod, with an 

 ovoid shell formed of hexagonal siliceous plates, and Micro- 

 gromia with a siliceous shell which is not made up of plates, 

 but is continuous. Closely allied to these, again, are the 

 Foraminifera, a very large group of rhizopodous Protozoa, 

 with calcareous shells, many of which are of great beauty and 

 complexity. The Foraminifera are all marine, and are ex- 

 tremely abundant. Some of them are pelagic — that is to say, 

 they float at or near the surface in the open ocean far away 

 from land ; and the bottoms of those oceans whose depth 

 does not exceed 2000 fathoms are generally covered with 

 deposits of a grey mud, which is chiefly composed of the 

 calcareous shells of the countless Foraminifera which have 

 lived and died in the waters above. At depths greater than 

 2000 fathoms, the proportion of Foraminifera in deep-sea 

 deposits becomes less and less, and at great depths the 

 deposits are largely composed of the siliceous skeletons of 

 another class of marine Rhizopods, the Radiolaria. In the 

 greater depths the fragile calcareous shells of the Foraminifera 

 have been dissolved by the action of the carbonic acid dis- 

 solved in the sea-water; it seems that the solvent action is 

 greater the greater the pressure. But the flinty skeletons of 

 the Radiolaria are not dissolved, and hence they take the 

 place of the Foraminifera at great depths. There are, of 



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