232 ON THE CLASSES OF THE 



like many of these last-mentioned animals, the Ascidia are 

 destitute of organs of locomotion; like these they have 

 their mouth at the bottom of the bag opposite to the tube 

 by which the water penetrates, and so placed that this 

 water cannot arrive at the intestinal canal without having 

 previously washed the surface of the branchiae. There 

 cannot therefore be the least doubt of our having now ar- 

 rived among the Mollusca, and nearer to the Acephalous 

 tribe than to any other or that divison. 



MOLLUSCA. 



The Acephala then, like the Jscidics, are soft particular 

 animals, without the principal organs of sense, and having 

 the mouth concealed and always destitute of teeth; de- 

 fects which force them to depend for subsistence on the 

 corpuscles, which the water may convey to the entrance 

 of their intestinal canal. The ample mantle no longer 

 forms a bag, but is composed of two great lobes, which 

 either opening in front envelop the body in the same man?- 

 ner as the cover of a book incloses its contents, or unit- 

 ing together in front forms a sort of tube open at one or 

 both ends. Between the two lobes of the mantle are the 

 branchiae no longer coating the sack, but composed of four 

 membranaceous thin semilunar plates, striated transversely 

 by the vessels on or between which the water passes. 

 The mantle is no longer of a cartilaginous nature, but now 

 is clothed with a bivalve calcareous shell. The heart, al- 

 most invisible at its first appearance among the Tunicata, 

 becomes in these animals less gelatinous and more di- 

 stinct. The blood goes to the heart from the branchiae, 

 and again from the heart by means of two arteries it is 

 dispersed over the body without the aid of another ven- 



