86 ZOOLOGICAL PHILOSOPHY 



order, because the large development of their mantle would have 

 prevented the functioning of these organs. 



In conformity with that law of nature which requires that every 

 organ permanently disused should imperceptibly deteriorate, become 

 reduced and finally disappear, the head, eyes, jaws, etc., have in fact 

 become extinct in the acephalic molluscs : we shall see elsewhere 

 many other examples of the same thing. 



In the invertebrates nature no longer finds in the internal parts 

 any support for muscular movement ; she has therefore supphed 

 the molluscs with a mantle for that purpose. Now the strength and 

 compactness of this mantle of the molluscs is proportional to the 

 necessity entailed by their locomotion and means of support. 



Thus in the cephalic molluscs, where there is more locomotion 

 than in those which have no head, the mantle is closer, thicker and 

 stronger ; and among the cephalic molluscs, those which are naked 

 (without shells) have in addition a cuirass in their mantle which is 

 stronger than the mantle itself and greatly facilitates the locomotion 

 and contraction of the animal (slugs). 



But if, instead of following the animal chain in the opposite direc- 

 tion from the actual order of nature, we followed it from the most 

 imperfect animals to the most perfect, we should then easily per- 

 ceive that nature when she was about to start the plan of organisation 

 of the vertebrates, was forced in the molluscs to abandon the use of a 

 crustaceous or horny skin as a support for muscular action, and to 

 prepare to transfer these fulcra into the interior of the animal. In 

 this way the molluscs are to some extent in the midst of this change 

 of system of organisation ; they have in consequence only feeble 

 powers of locomotive movements and they all carry out such move- 

 ments with remarkable slowness. 



CIRRHIPEDES. 



Animals without eyes which breathe by gills and have a mantle and jointed 

 arms with a horny skin. 



The cirrhipedes, of which only four genera ^ are yet known, should 

 be considered as a special class, since these animals cannot belong to 

 any other class of invertebrate animals. 



They approach the molluscs by their mantle and should be placed 

 immediately after the acephalic molluscs, since like them they have 

 neither head nor eyes. 



Yet the cirrhipedes cannot be a part of the class of molluscs ; for 

 ^ A natif a, Balanus, Coronula, and Tubicinflla 



