SHELLS AND MOLLUSCOUS ANIMALS. 183 



life, the animal is not the helpless creature which 

 we find hanging to the sea- weed. When young it 

 somewhat resembles the tadpole of the frog, though 

 it is extremely minute, and it swims actively in 

 the water, by quickly moving its tail. When in- 

 clined to fix itself to the rocks, a few fibres spring 

 from it, which form a holdfast, and it then gra- 

 dually changes into the condition in which it meets 

 our eye on the beach. Some of these Ascidians 

 are valued in China as articles of food, and are 

 probably relished by a people who seem to have a 

 singular taste in articles of diet, though their ap- 

 pearance is not such as would recommend them as 

 fit for the table. In their internal structure, they 

 much resemble the mollusks which form bivalve 

 shells, the flexible tunic, or case, being analogous 

 to the shell. 



There is one group of Ascidiae, which, unlike 

 the sea-squirts, are not single individuals, but 

 consist of a number of aggregated beings, forming 

 one living mass, in some measure resembling the 

 zoophytes, though they do not secrete either a 

 stony or horny framework. Some of the commoner 

 kinds may be found on our shores in plenty ; and 

 many of the tribe Botryllidae form a jelly-like 

 crust on some of our olive-green sea-weeds, or 

 other marine substances, almost covering the stems 

 of the marine plant, and presenting a beautiful 

 array of starry objects, of the most delicate blue 

 tints, or rich in every variety of red or purple, 

 or of all the bright hues of the glowing sunset. 

 These star-like objects on the surface of the jelly, 

 contain about ten or twelve individual creatures, 

 whose structure is found to resemble that of the 

 sea-squirt, and the jelly-like substance is common 



