34 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



in most cases yet another germinal layer, as the 



epiblast or hypoblast is respectively called, is developed 

 between the two we know already. It is appro- 

 priately spoken of as the mesol*last; it arises in 

 various modes, into the distinctions of which we need 

 not enter here ; it will suffice for us to know, that 

 in all the higher Metazoa the greater part of the 

 organism is fashioned out of it. 



In all cases the outer and inner layers undertake 

 the functions which their position entails on them ; 

 the cells of the epiblast become converted into the 

 parts which cover in and protect the rest of the body, 

 and give rise also to those organs by means of which 

 the organism becomes acquainted with what is going on 

 around it, sensory organs and nervous system. 

 The hypoblast remains always in connection with the 

 enteron, or digestive tract, forming the lining of its 

 walls, of the glands that are therein developed, and of 

 such outgrowths as may arise from it. In the lower 

 divisions of the Metazoa the mesoblast does not take 

 any large share in the formation of the organs ; it 

 remains in a more or less indifferent condition. In 

 the higher forms it becomes quite the most important 

 layer in the body, taking on as it does the duty of 

 developing the skeleton, the muscles, the blood, and 

 vascular system, the excretory organs, and the con- 

 necting tissues ; it always, also, becomes primarily 

 cleft or divided, so that a cavity is developed within 

 it ; this is the true body cavity, or ccelom, and all 

 animals that possess it may, whether they secondarily 

 lose it or not, be spoken of as the Cplomata. 



The acoalomate Metazoa are the sponges (Pori- 

 fera), and the great group to which belong hydra, 

 the jelly-fishes, and the sea-anemones (Ccelenterata). 

 The simplest sponges show hardly any advance on 

 the typical Gastrula, the amount of mesoblastic tissue 

 developed being small; but they are remarkable at 



