PAST HISTORY OF THE INVERTEBRATES 307 



exterior. There is a dorsal nerve cord and a ventral nerve 

 cord. A notochord-like structure extends into the middle 

 of the proboscis. 



A Synopsis of the Animal Kingdom 



The relative positions of the phyla are shown in the ac- 

 companying table. As regards the brief diagnosis of char- 

 acters listed for each class it is of course assumed that, in 

 addition, the statements made for the phylum apply to all 

 the members of each class within that phylum. Attention 

 should be called to the fact that in this table many of the 

 less important or less well-known classes have been omitted. 

 This applies especially to the Arthropoda and to the various 

 phyla of worm-like forms. 



The lowest chordates are sometimes called the Prochor- 

 dates. In the following table the phylum Chordata is in- 

 cluded. This makes it possible for us to have a complete 

 list of the phyla contained in the entire animal kingdom, 

 even though the higher chordates are not treated until later 

 in the book. 



PHYLA AND CLASSES OF THE ANIMAL KINGDOM 



Phylum I. Protozoa: chiefly single-celled animals. 



Class 1. Sarcodina: move by pseudopodia. Ex. Amoeba 



proteus. 

 Class 2. Mastigophora: move by flagella. Ex. Euglena 



viridis. 

 Class 3. Ciliata: move by cilia. Ex. Paramecium caudatum. 

 Class 4. Sporozoa: reproduce by spores, entirely parasitic. 



Ex. Plasmodium malarias. 

 Phylum II. Porifera (sponges) : body with numerous pores, no organs. 

 Class 1. Calcarea: with limy spicules. Ex. Grantia. 

 Class 2. Hexactinellida: with silicious spicules. Ex. Eu- 



plectella. 

 Class 3. Demospongia: with fibrous skeleton and frequently 



with silicious spicules. Ex. Heteromeyenia ryderi. 



