THE FIVE GROUPS OF ANIMALS. 121 



Cuvier presented them, or even in a more stringent sense, 

 making each grand division an absolute circumscription within 

 itself. Others look upon these four groups, or rather five groups, 

 since the Protozoa (Infusoria) are regarded at the present day 

 as a distinct division from the Zoophyta, as so many sub- 

 divisions which have more or less intimate relations with each 

 other; as if they were the components of a vast cloud which is 

 divided into five masses, having their edges mutually merged 

 into each other. 



In a certain accordance with this idea of the mutual relations 

 of the five divisions of the animal kingdom, I have constructed 

 these diagrams (figs. 55 to 64), to illustrate the corresponding 

 positions of the organs in the typical forms.* 



the feelers (/) ; aq\ aq~, aq*, ? 5 , o? 6 , ag 7 , the longitudinal aquiferous eanals 

 running close to the under surface of the skin; h, h l , the heart; c, the ribbon- 

 like nervous collar ; on, ov l , ov 2 , the reproductive organ ; 0, the external aper- 

 ture of ov. Original. 



* It may be objected here that some of the animals in these diagrams are 

 placed upside down, in order to bring the organs into corresponding position in 

 all of them ; but I would ask, then, do animals have any definite relation to up 

 and down ? Which, for instance, is the back among the Zoophytes ? The 

 Holothurians (Trepangs, fig. 54) creep on the side exactly opposite to that 

 on which the Sea-urchins do ! The latter creep in the position which the 

 diagrams (figs. 57, 58,) represent, i. e., heart downwards. Among the Mollusca, 

 the Cuttle-fish and Squid (ch. XL, fig. 124) swim usually backwards, and with the 

 back downwards. Now these comprise a large group among Mollusca. Another 

 considerable group among Mollusca, the so-called Nucleobranchiata, allied to 

 Conch-shells, swim rapidly in the sea back downwards. Many kinds allied to 

 the clams live in holes in the sand and mud, with the head downwards, and others, 

 like the oyster, rest on the side ! 



Among Articulata, who can say which is the back of those intestinal worms 

 (the Tape-worms, fig. 44) which live on the juices of the stomach of various 

 animals ? Even among the Insects, in which back and front seem to be most 

 distinctly marked, the Notonectas, water-bugs, invariably swim back downwards. 

 Of the Vertebrates, the Halibuts and Flounders creep and swim on the side. 

 The Bats rest all day hanging by their hind legs, head downwards. The Sloths, 

 a curious group of quadrupeds, always move among the branches of the trees, 

 where they constantly live, back downwards, hanging from the limbs by their 

 long claws. Now if it should be urged that the Sloths nevertheless have what is 

 to them their terra firma next the lower side of the body, we would ask, why 



