CHAPTER X. 



FORAMINIFERA, POLYCYSTINA, AND SPONGES. 



283. RETURNING now to the lowest or Rhizopod type of Animal 

 life ( 264), we have to direct our attention to three very remark- 

 able series of forms, almost exclusively marine, under which that 

 type manifests itself; all of them being distinguished by a skeleton 

 of greater or less density; and this skeleton being generally so 

 consolidated by mineral deposit, as to retain its form and inti- 

 mate structure, long after the animal to which it belonged has 

 ceased to live, even for those undefined periods in which they 

 have been imbedded as fossils in strata of various geological ages. 

 In the first of these groups, the Foraminifera, this skeleton con- 

 sists of a calcareous shell (save in a few exceptional cases, in 

 which the lorica remains flexible), that invests the sarcode body, 

 but is usually perforated with numerous minute apertures. In 

 the second group, also, the Polycystina, there is an investing 

 shell, perforated with numerous apertures ; but this shell is sili- 

 ceous, instead of calcareous. In the third group, on the other 

 hand, that of Porifera, or Sponges, the skeleton is usually 

 composed of a network of horny fibres, strengthened either by 

 calcareous or siliceous spicules, and having the soft animal body, 

 which is composed of an aggregate of Amoeba-like cells, in its 

 interstices : in this group, moreover, we have a departure from 

 the Rhizopod type, in the fact that certain parts of the free sur- 

 faces are furnished with cilia, whereby currents of water are 

 sustained, that serve both for nutrition and for respiration. 



284. Foraminifera. The beings now known under this desig- 

 nation, possess, for the most part, polythalamous or " rnany-cham- 

 bered" cells (Fig. 336), so strongly resembling those of Nautilus, 

 Spirula, and other Cephalopod Mollusks, that it is not surprising 

 that the older Naturalists, to whom the structure of these animals 

 was entirely unknown, ranked them under that class. As such 

 they were described by M. D'Orbigny (to whom we owe much of 

 our knowledge of this group), in all his earlier publications; and 

 they were distinguished from the ordinary Cephalopods that 

 possess a single siphon passing from chamber to chamber, by the 

 designation Foraminifera, which originally imported that the 

 communications between the chambers are commonly made by 



