PROGRESSION OF MOLLUSCOUS ANIMALS. 275 



animal world are modelled, will be hereafter shown 

 more at large. Yet, as, on the plan we are now 

 pursuing, the naturalist is to keep his mind free 

 from the influence of every theory, we only wish to 

 enforce upon him, from the above examples, the 

 necessity of making all such deviations from the 

 ordinary structure of the feet, a ground for separation 

 and distinction, even if no other exists, between two 

 forms in other respects perfectly similar. 



(187.) Hitherto we have contemplated those 

 animals only, which, with the exception of fish and 

 of serpents, are provided with articulated feet; but as 

 we descend to the more imperfectly or less organised 

 groups, as the Testacea, the JRadiata, and all those 

 " slimy things" which inhabit the depths of ocean, 

 no such organs exist, and locomotion is effected by 

 other means, and in various ways. Some of these 

 animals crawl like serpents upon their bellies, others 

 have little fleshy tubercles which in some measure 

 perform the office of feet : in the cuttle-fish, the long 

 processes which surround the head perform the 

 office of arms, feet, and fins : while the whole of the 

 Polypes, or corals, with many other groups, are de- 

 prived of all power of moving from the spot in 

 which they were born. These several peculiarities 

 enable us to frame essential characters for accurate 

 discrimination, of the most valuable description, 

 because they are not only obvious, but keep to- 

 gether the individuals of small but natural assem- 

 blages. External are always better than internal 

 distinctions ; for it is surely more desirable that we 

 should define an animal by something which every 



