346 The Animal Kingdom 



cavity and gradually tends to reduce the size of the blastocoel. In some 

 animals, the embryonic blastocoel is completely obliterated in the adult. 

 In these animals, the digestive cavity is the only cavity present, and 

 these animals are collectively known as the acoelomates. In other 

 phyla the blastocoel remains as a space between the body wall and the 

 digestive tract. This cavity is termed a pseudocoeJ, and the animals 

 possessing it are termed pseudocoelomates. Finally there are those 

 forms in which a true body cavity, the coelom, lined with tissue of 

 mesodermal origin is found. These are the coeJomate animals. 



Metamerism. — Among many of the higher animals, there is a 

 serial repetition of organs. This is usually accompanied by externally 

 visible grooves or divisions. This segmentation or metamerism is most 

 clearly visible in some of the annelid worms in which nearly every 

 segment bears paired appendages. Externally, metamerism is not so 

 evident among the vertebrates, but is the basis of construction. It can 

 be easily seen in the arrangement of some of the body muscles and in 

 the spinal nerves. 



Organ Systems. — Highly specialized organs are first seen in the 

 worm groups, and are present in all forms above these. The character of 

 these organs often is characteristic of certain of the phyla. Of particu- 

 lar importance in this regard are the excretory system, the nervous 

 system, and the circulatory system. 



THE PRINCIPAL DIVISIONS OF THE ANIMAL KINGDOM 



Subkingdom Protozoa. Animals consisting of but a single cell, or, if colonial, 

 without histological differentiation. 



Phylum Protozoa. Animals having the characteristics of the subking- 

 dom. 



Subkingdom Metazoa. Animals possessing many cells and having histological 

 differentiation. 



Branch A. Mesozoa. Cellular animals consisting of an outer cell layer 

 enclosing some reproductive cells. Endoparasites of many inverte- 

 brates. 



Phylum Mesozoa. Animals having the characteristics of the 

 branch. 

 Branch B. Parazoa. Animals of a cellular grade of construction with- 

 out organ systems, with the beginning of tissue formation, and with 

 internal cavities lined with flagellated cells. 



Phylum Porifera. Animals having the characteristics of the 

 branch. Example : sponges. 



