346 ON THE ORDERS 



those naturalists, by whom the MoUuscci are placed between 

 the Crustacea and Fishes, have not properly applied even 

 their own principles. If they think proper to found their 

 arrangement of animals almost entirely on the system of 

 circulation, we have a right to require of their consistency 

 that two classes, such as Crustacea and Fishes, having simi- 

 lar systems, shall not be separated by five classes of Elol- 

 lusca which have another system totally distinct. I had 

 before reason to contest the propriety of an application of 

 this principle of comparative anatomists to the arrange- 

 ment of Annulose animals ; and I do not even now regret 

 that they should have chosen to neglect it, since the in- 

 tervention of the ^4metabola between Crustacea and Fishes 

 has thus a sort of precedent. Indeed, out of the sphere 

 oi Fertehrata, the system of circulation, taken alone, rarely 

 deserves to have any great importance attached to it; and 

 we accordingly perceive that it has been overlooked even 

 by those naturalists who make it the ostensible principle 

 of their general distribution of animals. 



Vestiges of an ear have been detected in some few spe- 

 cies oi Crustacea; but as Hexapod insects, — which appear 

 to enjoy the sense of hearing much more ])erfectly, although 

 from the difference between the plan of their construction 

 and ours we are unable to discover the organ, — have not 

 the vestibulum of the Crustacea, the observation is of little 

 or no use towards determining the relative perfection of 

 the two groups. This is an argument indeed which has 

 been elsewhere urged ; but I would rather be taxed with 

 repetition than avoid calling the entomologist's most care- 

 ful attention to such an extraordinary point of anatomy. 



The Crustacea are remarkable for having two pair of 

 antennas, which are classed as external and internat. 



