AN INTRODUCTION TO THE METAZOA in 



Organogeny. Each germ layer gives rise to certain organs 

 of the adult animal. An organ may be defined as an association 

 of tissues having a definite form and special function. In any 

 of the more complex animals eight systems of organs can be recog- 

 nized: (i) digestive, (2) circulatory, (3) respiratory, (4) excre- 

 tory, (5) skeletal and integumentary, (6) reproductive, (7) mus- 

 cular, (8) nervous. The study of the origin of these organs from 

 the germ layers is known as organogeny. The organs derived 

 from the different germ layers may be briefly listed as follows. 

 From the ectoderm arise the epidermis, epithelium of various 

 organs, and the nervous system; from the mesoderm come the 

 muscles, connective and supporting tissues and blood and blood 

 vessels; the entoderm becomes the epithelium of the digestive 

 tract, pharynx, and respiratory tract. 



Ccelom. Figure 53, N, O shows one method of origin of the 

 coelom, a cavity of great importance in the bodies of many of the 

 complex Metazoa. Two pouches (N) formed from the inner 

 layer of the gastrula are pinched off and come to lie one on either 

 side of the alimentary canal (O). The cavities of these sacs 

 constitute the ccelom. By a lateral growth and a breaking 

 through at the ends of the sacs, the cavities unite to form a single 

 space between the alimentary canal and the body wall. All 

 Metazoons are frequently separated into two groups, the ccelo- 

 mata and the acalomata, according as the ccelom is present or 

 absent. Of the types considered in the following chapters 

 Hydra may be mentioned as an accelomate, the earthworm as 

 a ccelomate. 



Table III contains a series of diagrams and descriptions which 

 are intended to represent the methods of reproduction exhibited 

 by the types already studied, and to show those of certain Meta- 

 zoa as well. In order to make the diagrams as simple as possible 

 certain details have beeen omitted; nevertheless, it is believed 

 that a correct idea of the reproductive processes will be gained 

 in every case. 



