THE ANIMAL KINGDOM. 



METAZOA. 



MULTICELLULAR animals in which there is an ectoderm (=epi- or 

 hypo-dermis) and an endoderm with tissues of various kinds, sensory, 

 nervous, muscular, connective and reproductive, derived either solely from 

 them, or, with the exception of the two tissues first-named, from an inde- 

 pendent source. A Gastrula stage occurs in the life-history of the indi- 

 vidual. There are two subdivisions, the Coelomata and Coelenterata. 



COELOMATA. 



METAZOA, in which an intermediate layer of cells, the mesoblast or 

 mesoderm, developed in different ways (pp. xxviii-ix) intervenes between 

 the epi- and hypo-blast of the embryo. It gives rise to the muscular and 

 connective tissues, to the lymph or blood, to the nephridial (renal) and 

 with perhaps few exceptions to the reproductive cells. A cavity or a 

 system of cavities or channels, known as the coelome, is typically present ; 

 it lies within the mesoblast and is not homologous throughout the Coelo- 

 mata (pp. xxix xxx). In a few instances (most Turbellaria, the Trematoda, 

 and Cestoda\ it is represented by intercellular spaces. The principal axis 

 of the Gastrula passing through the blastopore, never persists as the main 

 axis of the body. Bilateral symmetry is always established. A shorter 

 anterior region or head which is preoral, and a longer postoral region, the 

 body, are readily distinguishable in most instances. The Echinodermata, 

 however, acquire a secondary radial symmetry. An anus is typically 

 present. 



The Coelomata include the phyla Chordata, Mollusca, Arthropoda, 

 Echinodermata, together with Vermes, as well as a few groups, the zoological 

 position of which is uncertain, e. g. Brachiopoda and Polyzoa. 



PHYLUM CHORDATA. 



BILATERALLY symmetrical coelomate Metazoa, in which the neural 

 surface of the body is dorsal, the haemal ventral, i.e. the surface of locomo- 

 tion, the reverse of what obtains in the other phyla of this division of Metazoa. 

 The body is segmented, except in Urochorda, where segmentation is ob- 

 scurely indicated in the caudal region alone. The central nervous system 

 is formed typically from a neural (medullary) plate of epiblast which 



