272 



On the Classification of Animals 



looking this principle lias often led authors to allow too great 

 importance to the structural differences among inferior or degra- 

 dational groups. 



XII. Under any class, order, tribe, the typical groups are often 

 represented more or less clearly among the subdivisions of the 

 degradational. Hence characteristics which separate the typical 

 groups frequently separate only subordinate divisions under an 

 inferior or degradational group. Examples occur in the class of 

 Fishes under Vertebrates, in whose subdivisions the other classes 

 of Vertebrates are partly represented ; in the order of Ootocoids 

 under Mammals, which has its megasthenic and microsthenic 

 subdivisions ; under Worms, &c. 



4. Distinction between Animals and Plants. 



XIII. This subject well illustrates a fundamental distinction 

 between animals and plants. 



a. An animal, as has been stated on page 94, has fore-and-aft, 

 or antero-posterior, polarity ; that is, it has a fore-extremity and 

 a hind-extremity which have that degree of oppositeness that char- 

 acterizes polarity. 



h. With this fore-and-aft polarity there is also dorso-ventral 

 polarity. 



c. The dorso-ventral and antero-posterior axes are at right 

 angles to one another. In Invertebrates and a large part of 

 Vertebrates the antero-posterior axis is horizontal and the dorso- 

 ventral vertical ; and only in Man, the prince of Mammals, is the 

 former vertical and the latter horizontal. 



d. An animal, again, has not only oppositeness between the 

 fore- extremity and hind-extremity, but also a head, the seat of 

 the senses and mouth, situated at the fore-extremity and con- 

 stituting this extremity. 



e. In addition, the typical animal is forward moving. 



But in animals of the inferior type of Radiates, while there is 

 an anterior and a posterior side, and also, in most species, forward 

 motion, the mouth-aperture — which indicates the primary centre 

 in an animal (p. 82) — is not placed at one extremity, but is more 

 or less nearly central ; and almost precisely central in the sym- 

 metrical (and therefore inferior) Kadiates. The mouth-extremity 

 and the opposite are at the poles of the dorso-ventral axis, and not 

 at those of the antero-posterior ; that is, they are at the extremity 

 of the axis which in the inferior animals is normally vertical. 

 This is true even in a Holothuria, the mouth of which is not at 

 the anterior extremity, but is central, or nearly so, as in an 

 Echinus. A Limulus has been referred to on page 90 as show- 

 ing an approximation, under the true animal type, to this same 

 central position of the mouth. 



