PORIFERA. 



'3 



be a tubular polype, or a more or less bell-like " medusoid," 

 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 

 perforating the walls, 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," or both siliceous and horny at once. Water 

 passes in by the small inhalant pores, and out by the 

 exhalant aperture. 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 consist 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 ways to form a "body." But 

 the Protozoa form no " body " ; they remain 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 (&) there are 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, nor sexual 

 reproduction in the ordi- 

 nary sense of the phrase. 

 The series includes — „ _ ., „ 



Fig. 17. — Fossil Foramimfera, 

 (a) Infusorians, with actively Nummulites. 



moving lashes of living matter. 



{6) Rhizopods, with outflowing threads or processes of living matter, 

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



(c) Sporozoa, parasitic forms, without either lashes or outflowing 

 processes. 



