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 Jestacea, the Radiata, 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 craw] 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 
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