GENERAL CLASSIFICATION 125 



RECOGNITION GROUP CHARACTERS 



The commoner members of the several branches may be recog- 

 nized by the following characters : — 



1. The Protozoans are the only one-celled animals. 



2. The Sponges are the only animals having pores all over the 

 body for the inflow of water. 



3. The Polyps are the only many-celled animals having a single 

 opening into the body, serving for both mouth and vent. They 

 are radiate in structure, and usually possess tentacles. 



4. The Echinoderms are "marine animals of more or less radiate 

 appearance, having a food tube in the body separate from the 

 body wall. 



The following groups are plainly bilateral : that is, dorsal and 

 ventral surfaces, front and hind ends are different. 



5. The Vermes have usually a segmented body but lack jointed 

 legs. 



6. The Arthropods ' have an external skeleton and jointed legs. 



7. The Molluscs have soft bodies, no legs, no skeleton, but 

 usually a limy shell. 



8. The Vertebrates have an internal skeleton of bones, and 

 usually two pairs of legs. 



1 Insects and crustaceans. 



Fig. 247. — A Snail. (Which 

 branch ? Why?) 



