590 PHILOSOPHY OF XOOI.OCiY. 



larger animals, will occupy the second. Objections to the 

 formation of this division may be advanced, from the con- 

 sideration, that the characters employed in arrangement 

 ought not to be taken from station, but from structure, so 

 that unless the internal animals exhibit some peculiarity in 

 their organization, distinguishing them from the external 

 ones, they should not occupy a division apart, but be intro- 

 duced according to their analogies among the groups of 

 external genera. To this it may be answered, that while 

 the animals of the division Entozoa cannot be characterised 

 by any peculiarity of structure, nay, while they differ re- 

 markably from one another, we draw the conclusion, that 

 they must necessarily differ from the external groups in 

 the structure and function of their organs of respiration, 

 since they can neither possess gills nor lungs of the ordi- 

 nary kinds. But, after all, the systematical convenience of 

 the division (a principle frequently acted upon, but seldom 

 acknowledged), is the best apology for its employment. 



Order I. 

 Habitation external. 



Section I. CIRRHIPIDES. 



Body protected by a multivalvular shell. 



This group includes those animals which constitute 

 the Linnean genus Lepas. They are all sessile animals, 

 and the base by which they are attached corresponds with 

 the coronal or anterior aspect of the body. The tunic is 

 closed laterally, dorsally, and anteriorly, but is more or less 

 separated at the ventral aspect. The tunic is strengthened 

 by testaceous plates, to which the body adheres by one or 

 more muscles. The head appears as a slight eminence at- 

 tached to the thorax. The thorax is followed by an abdo- 

 men supporting six feet on both sides, each of which con- 



