NATURAL SYSTEMS. — MACLEAY’S. 209 
of strong analogy. The groups in question have so long 
engaged our attention, that we shall venture now to ex- 
press our opinions upon them. The intestinal worms 
(Intestina) do not appear to enter into the circle of the 
Acrita, but rather represent that class among the An- 
nulosa, where they follow, in close affinity, the Annelida 
or red-blooded worms. The Cirripeda, which enter 
also among the Annulosa, may possibly represent the 
Branchiopod Mollusca; but we think this very doubtful : 
while the analogy between the reptiles and Ornithorhyn- 
chus is neither direct nor natural, seeing that it is by 
this latter form that the Mammalia are connected in 
the most satisfactory manner to birds. If any na- 
turalist should be inclined to doubt this latter theory, 
he will be fully convinced of its accuracy by analysing 
the class of Mammalia to which it belongs. If, however, 
Mr. MacLeay had merely said that the opposite points 
of a circle always evince a strong analogy to each other, 
he would only have illustrated one of his own propo- 
sitions ; for this resemblance follows, as a matter of 
course, if the contents of one circle show parallel rela- 
tions of analogy to the contents of another circle. 
(263.) The fifth proposition of our author, and 
which he considers as one of the characteristics of natural 
groups, is this, — that one of the five larger groups in 
every circle contains representations of all the others. 
An instance of this will best explain to the reader his 
full meaning. Mr. MacLeay considers that the polypes 
(Acrita) form one of the five great classes or groups of 
the animal circle ; and he proceeds to explain how this 
group contains types of all the others, together with one 
peculiar to itself. ‘* At first sight,” he observes, “ there 
appears to be a want of that symmetry in this circle 
(the Acrita), so observable in the others which compose 
the great divisions of the animal kingdom; for the Ra- 
diata (or star-fish) have all a classical type to which 
their several structures may be referred, as also have 
the Annulosa, Vertebrata, and Mollusca ; whereas here 
we see nature choosing every possible type of form, and 
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