PORIFERA. 



and in some cases the two forms are included in one life 

 cycle. Budding is very common, and many of the sedentary 

 forms " corals " -have shells of lime. 



Porifera. Sponges, or Porifera, are the simplest many- 

 celled animals. In the simplest forms, the body is a 

 tubular, two-layered sac, with numerous inhalant pores by 

 which water passes in, with a central cavity lined by cells 

 bearing lashes or flagella, and with an exhalant aperture. 

 But budding, folding, and other complications arise, and 

 there is almost always a skeleton, calcareous, siliceous, or 

 " horny." With few exceptions they are marine. 



All the animals hitherto mentioned have bodies built up of many cells 

 or unit masses of living matter ; but there are other animals, each of 

 which consists of a single cell. These simplest animals are called Protozoa. 



Every animal hitherto mentioned, from mammal or bird to sponge, 

 develops, when reproduction takes its usual course, from a fertilised 

 egg - cell. This egg - cell or ovum divides and redivides, and the 

 daughter cells are arranged in various way-- to form a "body." But 

 the Protozoa form no " body ' : ; they remain (with few exceptions) 

 single cells, and when they divide, the daughter cells almost invariably 

 go apart as independent organisms. 



Here, then, is the greatest gulf which we have hitherto noticed 

 that between multicellular animals (Metazoa) and unicellular animals 

 (Protozoa). But the gulf was bridged, and traces of the bridge remain. 

 For (a) there are a lew Protozoa which form loose colonies of cells, 

 and (/>) there are a few multicellular animals of great simplicity. 



Protozoa. - The Pro- 

 tozoa remain single cells, 

 with few r exceptions. Thus 

 they form no " body " ; and 

 necessarily, therefore, they 

 have no organs in the ordi- 

 nary sense. They illustrate 

 the beginnings of sexual 

 reproduction, and they are 

 not subject to natural death 

 in the same degree as 

 Metazoa are. The series 

 includes- 



(a) Infusorians, with actively moving lashes of living matter. 

 (/>) Rhizopods, with outflowing threads or processes of living matter, 

 e.g. the chalk-forming Foraminifera (Fig. 17). 



(c) Sporozoa, parasitic forms, usually without either lashes or out- 

 flowing processes. 



FIG. 17. - - Fossil Foraminifera 

 (Nummulites) in limestone. 

 After Zittel. 



