i88 TUNICARIES. 



cially those of the water-lily and the bistort upon floating wood, 

 and upon the stems of various plants. In order to examine these 

 beautiful organisms in a living state, it is only necessary to allow 

 the leaf, or other substance to which they are attached, to remain 

 for some little time undisturbed in a glass of clear water, when 

 they will be soon seen, spreading forth their beautiful tentacula, 

 as they protrude from their delicate cells ; and by frequently 

 changing the water they may be kept alive for months, affording 

 objects of continual interest for microscopical observation. 



When thus examined, it will be seen that the fresh-water Polypes 

 differ from the marine species in the arrangement of their ten- 

 tacula. In the latter, as we have seen in Bbwerbankia (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 crescentic or horse-shoe-shaped 

 series (Fig. 187). 



CHAPTER XVI. 



SECOND CLASS OF MOLLUSKS. 

 TUNICARIES. TUNICATA.* 



r I ^HE name of this class is derived from the circumstance that 

 JL 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, 

 /""' v -x suc h as a stone, a shell, or a piece 



of wood ; while above it is pro- 

 vided with two orifices, one appro- 

 priated to the entrance, the other 

 to the discharge of the surround- 

 ing water. 



Various are the forms under 

 which these creatures present 

 themselves to the naturalist ; from 

 among which we will select one of 

 the simplest for special descrip- 



FIG. 1 38. EXTERNAL FORM OF ASCIDIA. , 



tion. 



* Tunica, a coat: so called because they are enclosed in a leathery-looking tit) 



