232 GENERAL HISTOEY OF THE INFUSOEIA. 



minifera become still more multiplied, and many previously unobserved genera 

 make their appearance. In the Silurian and Devonian rocks of the palaeozoic 

 series, Foraminifera appear to be absent. In the carboniferous deposits 

 D'Orbigny found one species, but detected none in the Permian, Triassic, or 

 Jui'assic strata. Mr. King has, however, discovered shells in the Permian rocks. 



Many genera have hitherto been found only in the fossil state : some such 

 we may suppose to have become extinct ; but others will probably be discovered 

 when the search after hving specimens is further prosecuted. It may be 

 generally stated that the relative number of identical fossil and recent species 

 is much greater in this family of Foraminifera than in any other known ; and 

 specific forms have continued from the Mesozoic era until the present day, so 

 connecting, as by an imbroken chain, the fauna of our own time and that of 

 almost countless ages past. 



QuESTION^ OF THE CeLL-NATUEE OP EhIZOPODA, AND OF THE ChAEACTER OF 



Foraminifera as Individuals, or as Colonies of Aniiials. — The prevailing 

 theory of the cellular composition of all animal and vegetable tissues induced 

 several distinguished naturalists to represent the Rhizopoda as ceUs. KoUiker 

 ingeniously argued (J. M. S. 1853, i. p. 101) in favour of this view, and for 

 a time succeeded in persuading most scientific men of its truth. It had the 

 character of a grand generalization, and recommended itself by its simphcity. 

 Yarious structural peculiarities and general considerations are, however, 

 opposed to this theory: these we will adduce after KoUiker's arguments 

 have been stated. He first assumes that the Rhizopoda and Ciliated Pro- 

 tozoa are comprehended in a single class of simple animals, which, like the 

 Gregarince, are unicellular ; and he foi^ther groups the Actinophryina with 

 Rhizopoda. The absence of an integument to represent the cell-wall, and 

 in most of them of a recognized nucleus, are difficulties he would explain 

 away. Pirst, he supposes that, where a nucleus is not seen, it " may have 

 existed at an earlier period, and be absent only in the full-gro^vn animal, or, 

 again, that it may be entirely wanting, and stiU the animal be regarded as a 

 cell." Secondly, " with respect to the membrane, it may be regarded as certain 

 that there are cells mth a membrane of such extreme tenuity as to be hardly 

 distinguishable from the contents," and others in which at a later period all 

 difference between the membrane and contents disappears, — for instance, the 

 elements of the smooth muscles of the higher animals." AATiich of these two 

 possible conditions obtaias in the Rhizopods, he cannot undertake to say, but 

 would remark '' that their other relations are not opposed to the notion that 

 they may be simple cells, — such as their stnictureless homogeneous contents, 

 their contractihty, and the vacuoles which occur in them, resembling in all 

 respects the contents of the body of unicellular Infusoria. So, likewise, the 

 simplicity of their form and mode of taking food, so closely resembhng the 

 way in which Infusoria introduce a morsel into their parenchyma. Certainly 

 the presence of a ceU-membrane is scarcely reconcHeable vrith the circumstance 

 that the body is capable of admitting a morsel of food at any part of the sur- 

 face ; but in one point of view it is not indispensably necessary to assume 

 that such exists in the fully-developed Actinoplirys, and in another it is by 

 no means wonderful that a membrane, in consistence almost the same as the 

 rest of the parenchyma, should be capable of being torn and of reuniting." 

 It is therefore, he concludes, best to consider the Rhizopoda simple, although 

 modified, cells, especially since there is little else to be made of them. " It 

 cannot be admitted that they consist of a whole aggregation of cells ; and as 

 little is it to be supposed that they are simply a mass of animal matter with- 

 out further distinction — as it were, independent hving ceU-contents. And 

 the less can this opinion be entertained, because " cells are the elementary 



