PORIFERA. 



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." Apart from one family (Spongillidse), all sponges 

 are marine. 



Contrast of Metazoa and Protozoa. All the animals hitherto 

 mentioned have bodies built up of many cells, 1 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 cohere and are differentiated 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 few Protozoa which form loose colonies of cells, 

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



Protozoa. The Pro- 

 tozoa remain single cells, 

 with few exceptions. Thus 

 they form no " body " ; 

 and necessarily, therefore, 

 they have no organs in 

 the ordinary sense. They 

 illustrate the beginnings of 

 sexual reproduction, and 



they are not ^ subject to FlG> I7< _ Fossil Foramm if er a 

 natural death in the same (Nummulites) in limestone. 



degree as Metazoa are. After Zittel. 



The series includes 



1 A cell may be defined as a unit corpuscle or unit area of living 

 matter, typically controlled by a single nucleus, 



