16 ZOOLOGY. 



a still earlier generation. The older observers have 

 placed the ofispring and the parents, and sometimes 

 the grandparents too, of the same animal species in 

 different species, or even genera or families, until 

 newer researches on the reproduction and development 

 of these animals have proved them to belong to one 

 and the same species (see Fig. 7 and explanation). 

 The method of reproduction by which a species appears 

 in two or several forms is distinguished as heterogeny 

 and metagenesis, or alternation of generations. In 

 the first (Fig. 7) sexually reproducing animals alternate 

 with other sexual animals. It may be that these are 

 of separate sexes, or else they may possess both male 

 and female organs (hermaphrodMe). In metagenesis 

 a sexual generation regularly alternates with one or 

 several generations reproducing asexually. 



The animal kingdom falls (cf. p. 2) into sub- 

 kingdoms or main divisions. Seven of these are 

 commonly distinguished : I. Backboned animals ; II. 

 Jointed-limbed animals ; III. Worms ; IV. Molluscs ; V. 

 Echinoderms ; VI. Coelenterates ; VII. Protozoa. 



First Sub-Kingdom : VERTEBRATA (Backboned 

 Animals). 



The Vertebrate body possesses a bilateral or two- 

 sided symmetry ; i.e. it can be separated into two 

 exactly corresponding halves, by a plane of division. 

 The bilateral symmetry is strictly carried out as 

 regards the external parts of the body, a single 

 exception to this being flat-fish (plaice, flounder, etc.) ; 

 but, on the other hand, it is more or less obliterated 

 in the arrangement of the internal organs. In the 

 Vertebrate body we find, as an axis, a vertebral column 

 (backbone) made up of vertebrae, and traversed by 

 the vertebral canal. As soon as this canal widens out 



