A GENERAL REVIEW OF THE ANIMAL KINGDOM I49 



entering a central cloaca. This is in reality not a true digestive 

 tract. It communicates with the exterior by one or more large 

 passages. They are attached and often form large colonies by 

 budding. 



188. Phylum Protozoa {first animals). — ^AU the preceding 

 phyla of animals consist, 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 §55), 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. 



189. 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 gUls. 



Throughout life Class Pisces. 



In embryonic life only (with a few exceptions), 



Class Amphibia. 

 Do not possess functional gills. 



Epidermal covering of scales Class Reptilia. 



Epidermal covering of feathers Class Aves. 



Epidermal covering of hair Class Mammalia. 



Without notochord Invertebrata. 



Bilaterally symmetrical (chiefly). 

 Body segmented. 



Paired jointed appendages present Phylum Arihropoda. 



Paired, jointed appendages absent Phylum Annelida. 



Body unsegmented; without paired appendages. 

 With mantle — often secreting shell. 



Phylum Mollusca. 



No mantle Unsegmented Worms. 



Radially symmetrical in adult Phylum Echinodermata. 



Without true ccelom. 



With a single mouth, which also functions as an anus: stinging cells 



(usually) Phylum Ccelenterata. 



With numerous incurrent openings or pores, and only one — or few — 



excurrent. No stinging cells Phylum Porifera. 



Single-celled animals (chiefly) Phylum Protozoa. 



