RECOGNITION GROUP CHARACTERS
The commoner members of the several branches may be recognized 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[[3]] have an external skeleton and jointed legs.
[3]. Insects and crustaceans.
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.
Fig. 247.—A Snail. (Which branch? Why?)
CHAPTER XI
BATRACHIA
The theory of evolution teaches that animal life began in a very simple form in the sea, and that afterward the higher sea animals lost their gills and developed lungs and legs and came out to live upon the land; truly a marvellous procedure, and incredible to many, although the process is repeated every spring in countless instances in pond and brook.
In popular language, every cold-blooded vertebrate breathing with lungs is called a reptile. The name reptile is properly applied only to lizards, snakes, turtles, and alligators. The common mistake of speaking of frogs and salamanders as reptiles arises from considering them only in their adult condition. Reptiles hatch from the egg as tiny reptiles resembling the adult forms; frogs and salamanders, as every one knows, leave the egg in the form of tadpoles (Fig. [248]). The fact that frogs and salamanders begin active life as fishes, breathing by gills, serves to distinguish them from other cold-blooded animals, and causes naturalists to place them in a separate class, called batrachia (twice breather) or amphibia (double life).