326 THE CAUSES OF THE XI 
8 
and which we technically call Vertebrata. But 
there are multitudes of other animals, such as 
crabs, lobsters, spiders, and so on, which we term 
Annulosa. In these I could not point out to you the 
parts that correspond with those of the horse,— 
the backbone, for instance,—as they are constructed 
upon a very different principle, which is als 
common to all of them; that is to say, the lobster, 
the spider, and the centipede, have a common 
plan running through their whole arrangement, 
in just the same way that the horse, the dog; 
and the porpoise assimilate to each other. 7 
Yet other creatures—whelks, cuttlefishes, 
oysters, snails, and all their tribe (JMollwseca)—_ 
resemble one another in the same way, but differ 
from both Vertebrata and Annulosa ; and the like 
is true of the animals called Cwlenterata (Polypes) 
and Protozoa (animalcules and sponges). § 
Now, by pursuing this sort of comparison, 
naturalists have arrived at the conviction that 
there are,—some think five, and some seven,—but 
certainly not more than the latter number—and_ 
perhaps it is simpler to assume five—distinct plans | : 
or constructions in the whole of the animal world 5 
and that the hundreds of thousands of species 
of creatures on the surface of the earth, are all 
reducible to those five, or, at most, seven, plans of 
organisation. | ; 
But can we go no further than that? When 
one has got-so far, one is tempted to go on as 
