40 AN INTRODUCTION TO ZOOLOGY. 



tlirougli the intestine ; lobsters have gills attached to 

 their legs. Whatever the form of the respiratory 

 organ, it is always such as to bring the blood to a 

 thin layer of cellsj through which oxygen can pass 

 from the air or water; this layer is folded so as to 

 afford as large a surface as possible. The breathing 

 organs of veitebrates are exceptional in being derived 

 from the hypoblast, those of invertebrates being 

 usually derived from the skin-layer. 



Among parts derived from the hypoblast must be 

 mentioned the notochord of vertebrates, a structure 

 of which more will be said hereafter (see p. 76). 



The space between the alimentary canal and the 

 body wall is the body cavity, or ccelom, as it is 

 variously called (Fig. 12). It is a very important fea- 

 ture in the arrangement of the internal organs ; and 

 its nature can only be understood from a study of its 

 arrangement in primitive animal types, and of the way 

 in which it is formed in the embryo. The hydroid 

 animals {Ccelenterata) , instead of having an alimentary 

 canal lying in coils inside a body cavity, a tube 

 fastened within a hollow space, have only a simple 

 cavity (Fig. 11). The early embryonic stages of many 

 animals are formed like this, and the cavity is called 

 the archenteron, or primitive stomach; the cells 

 which line it are the hypoblast or stomach layer. In 

 some cases it appears that this archenteron afterwards 

 branches into two other portions besides the main 

 one, and these two gradually extend till they go all 

 round the main portion, so that the whole arriinge- 



