482 



CHAPTER X. 



FORAMINIFERA, POLTCTSTINA, AND SPONGES. 



365. Returning now to the lowest or Rhizopocl type of Animal 

 life (§ 326), 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 distinguished by Skeletons of 

 greater or less density, and these skeletons generally so consolidated 

 by Mineral deposit as to retain their form and intimate structure 

 long after the Animals to which they belonged have ceased to live, 

 even for those undefined periods in which they have been im- 

 bedded as Fossils in strata of various Geological ages. In the first 

 of these groups, the Foraminifera, the Skeleton usually consists of 

 a Calcareous many-chambered Shell, which closely invests the 

 Sarcode-body, and which, in a large proportion of the group, is 

 perforated with numerous minute apertures ; this shell, however, 

 is sometimes replaced by a ' Test ' formed of minute grains of Sand 

 cemented together ; and there are a few cases in which the Animal 

 has no other protection than a Membi'anous envelope. — In the 

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

 perforated with apertures ; but this shell is Siliceous, and has 

 usually but one chamber ; and its apertures are often so large and 

 numerous, that the solid portion of the shell forms little more than 

 a network, thus indicating a transition to the succeeding group. 

 — In the group of Porifera or Sponges, the Skeleton is usually 

 composed of a network of Horny fibres, strengthened either by 

 Calcareous or by Siliceous Spicules, and having the soft Animal 

 substance, which is composed of an aggregate of Amoeba-like 

 bodies, in its interstices : in this group, moreover, we have a de- 

 parture from the Rhizopod type, in the fact that certain parts 

 of the free surfaces are furnished with Cilia, whereby currents 

 of water are maintained, that serve both for Nutrition and for 

 Respiration. 



366. Foraminifera. — The beings now known under this de- 

 signation possess, for the most part, Polythalamous or ' many- 

 chambered ' Shells (Plate xv.), often 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 



