140 ZOOLOGY. 



are more or less tubular, sac-shaped animals often attached by 

 one end, with the mouth, which also functions as the anus, at 

 the other surrounded by clusters of tentacles. Many secrete 

 skeletons, and some form immense attached colonies. 



182. Porifera. This group, to which belong the sponges, 

 is sometimes classed with the Coelenterata. While similar to 

 them in habit the sponges are much less highly organized and 

 unified. Instead of a single mouth opening into the digestive 

 tract, sponges have many openings or pores (whence the name 

 Porifera) which are the beginnings of tubes entering a central 

 cloaca or sewer. This is in reality not a true digestive tract. 

 It communicates with the exterior by one or more large pas- 

 sages. They are attached and usually form large colonies by 

 budding. 



183. Protozoa. All the preceding phyla of animals con- 

 sist, in the adult stage, of many cells among which there is 

 more or less differentiation. In all of them the adult passes 

 through stages in which the cells are arranged in at least two 

 layers (ectoderm and entoderm^ see 53), from which the 

 tissue-masses arise. These animals are known as Metazoa. 

 In the remaining phylum the Protozoa the animals are 

 single cells, or at most loose aggregations of similar cells. 

 They are the lowest of animals and are for the most part in- 

 visible to the naked eye. 



184. An Artificial Key to the Phyla of the Animal Kingdom. 



Many-celled animals METAZOA. 



With true coelom Coelomata. 



Possessing notochord (and often vertebral column), 



Phylum Chordata. 

 Possess functional gills. 



Throughout life Class Fishes. 



In embryonic life only (with a few exceptions), 



Class Amphibia. 

 Do not possess functional gills. 



Epidermal covering of scales Class Reptiles. 



Epidermal covering of feathers Class Birds. 



Epidermal covering of hair Class Mammals. 



