254 CONCHIFERA. 



of vitality. A closer and microscopic inspection, 

 however, will soon show us currents in the water sur- 

 rounding them, streams rejected from their apertures, 

 and water rushing in, indicating that, however torpid 

 the creature may appear externally, all the machinery 

 of life, the respiratory wheels and circulatory pumps, 

 are hard at work in its numerous recesses. The 



FIG. 192. COMPOUND ASCIDIAX. STARRY BoiRYLLus .* a, natural size; b, one 

 of the composite stars magnified. 



whole mass, in fact, is composed of an aggregation 

 of minute Ascidians, conjoined in elegant microscopic 

 groups, all constructed upon the same plan as that 

 described above, and all actively employed in taking in 

 and ejecting the currents that bring them nutriment. 



CHAPTEK XVI. 



THIRD CLASS OF MOLLUSCA. 



CONCHIFERA.t 



THE inhabitants of bivalve shells constitute a very 

 numerous and important class. Encased in dense 

 and massive coverings, of such construction as to 

 preclude the possibility of their maintaining more 

 than a very imperfect intercourse with the external 

 world, and deprived even of the means of communi- 

 cation with each other, we might naturally expect 

 their organization to correspond in its general feeble- 

 ness with the circumscribed means of enjoyment, and 



* &6rpvs, botrys, a bunch of grapes. 

 f Concha, a shell ; fero, I carry. 



