172 A COURSE ON ZOOLOGY. 



into two groups, the salpae and the ascidians. Some of 

 the latter are simple, some are compound, that is. they 

 are found singly or in numerous colonies. Their form, 

 generally speaking, is that of a tube having two orifices 

 corresponding to the two extremities of the alimentary 

 canal. They respire by means of an apparatus like 



FIG. 100. 



TEREDO IN ITS BOEING. 



internal gills. The salpse have alternate generation. A 

 single individual produces a chain-like colony, and each 

 of these produces a single detached individual that in 

 its turn produces a chain of successors. 



The polyzoans, and ascidians, are aquatic creatures, 

 some living in fresh water, others in the sea. They have 

 vibratile tentacles, and usually dwell in colonies. 



CHAPTER XIX. 

 Radiates. 



UNDER the name radiates are grouped a large number 

 of lower creatures whose bodies present, in a more or 

 less apparent manner, the form of a star ; some, how- 

 ever, are globe-shaped, some cylindrical. 



