Logax.] The Invertebrates, Introduction. 437 



General Description of Subkingdoms. 



The animal kingdom is divided into eight subkingdoms. 

 Named in order of their development these subkingdoms are : 

 (1) Protozoa. (2) Ccelenterata. (3) Echinodermata. (4) 

 Vermes. (5) Molluscoidea. (6) Mollusca. (7) Arthropoda. 

 (8) Vertebrata. The first seven divisions comprise the inver- 

 tebrate animals, and the eighth or last the vertebrate animals. 

 All of these subkingdoms are represented by fossil forms in the 

 Upper Cretaceous of Kansas except the fifth. In abundance of 

 individuals Protozoa is the best represented, as the large chalk 

 beds of the Niobrara division are largely composed of these 

 forms. Diversity of species is undoubtedly best represented by 

 the Mollusca. In order that the general reader may have a 

 better understanding of the following systematic classification, 

 I have added a short description to the principal subkingdoms. 

 For description of the Protozoa, see Part VII. 



Coelenterata. 



Coelenterates are aquatic animals which vary much in form, 

 structure, and size. They are either free and independent, or 

 composite and attached. The body is composed of three modi- 

 fied layers of cells. The lowest coelenterates belong to a class 

 called Porifera. To this class belong the sponges, hydras, sea- 

 anemones and other forms. The sponges have a skeleton and 

 are attached to some submarine object. The horny or flinty 

 skeleton of sponges possesses an internal and external cellular 

 layer. The internal cells have cilia, the whip-like action of 

 which drives the water through the skeletal canals, allowing 

 the cells to grasp their food and oxygen from the flowing water. 

 The external cells can easily procure their food and oxygen 

 from the water which is in continuous circulation about the 

 sponge. The fresh-water hydra attaches itself mouth down- 

 wards to objects in the water. A number of tentacles extend 



