INVERTEBRA TES. 



Ccelentera. 



This series includes jelly fish, sea anemones, corals, zoo- 

 phytes, and the like, most of which .are equipped with 

 stinging cells, by means of which they paralyse their prey. 

 All but four or five are marine. The body may 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. 



Port/era. 



Sponges, or Porifera, are the simplest many celled animals. 

 In the simplest forms, the body is a tubular, two layered 

 sac, with numerous inhalent pores perforating the walls, with 

 a central cavity lined by cells bearing lashes or flagella, and 

 with an exhalent 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 inhalent pores 

 and out by the exhalent 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 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 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 organisms (Metazoa) and unicellular 

 organisms (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 multicellular organisms of great 

 simplicity. 



Protozoa. 



The Protozoa remain single cells, with few exceptions. 

 Thus they form no "body;" and necessarily therefore they 



