TUNICATA. 249 



water Polypes differ from the marine species in the 

 arrangement of their tentacula. In the latter, as 

 we have seen in Bowerbankia (Fig. 185), the tentacles 

 are disposed in an uninterrupted series around the 

 mouth, so as to resemble a funnel, whereas in the 

 fresh-water species, they are arranged in a crescentie 

 or horse-shoe-shaped series (Fig. 187). 



CHAPTER XV. 



SECOND CLASS OF MOLLUSCA. 

 TUNICATA.* 



THE name of this class is derived from the circum- 

 stance that the animals belonging to it are enclosed 

 in a tough, gristly or leathery bag, the lower 

 extremity of which is generally affixed to some 

 extraneous substance, such as a stone, a shell, or a 

 piece of wood ; while above it is provided with two 

 orifices, one appropriated to the entrance, the other 

 to the discharge of the surrounding water. 



Various are the forms under which these creatures 

 present themselves to the naturalist; from among 



FlG. 188. EXTERNAL FORM OF ASCIDIA. 



* Tunica, a coat, so called because they are enclosed in a leathery 

 looking tunic. 



M 3 



