22 CLASSIFICATION 



so-called irregular flowers. The two halves, conveniently termed 

 right and left, are not exact counterparts, but are what has been 

 termed mirror-images. That is to say, if one half be placed with 

 its cut surface against a mirror its reflection will correspond to 

 the missing half. But bilateral symmetry in an animal involves 

 more than a distinction between right and left, it means also, 

 or the terms right and left could not be properly used, a dis- 

 tinction between front (anterior) and back (posterior) ends. Not 

 only so, but we further perceive that an upper (dorsal) surface 

 can be recognized as distinct from an under (ventral) surface. 

 These points come out somewhat more clearly in lower animals 



Fig. I. — Cod-fish {Gadus tTtorrhtta) 

 The pelvic fins are seen under the throat and a pectoral fin on side of body above them. 



than in man. Take, for instance, a fish, say a cod (fig. i). The 

 anterior end is clearly the head-end, the end which under ordi- 

 nary circumstances goes first, and which, therefore, naturally 

 bears the mouth, and is provided with the most important 

 sense organs, such as those of smell, sight, and hearing — here 

 most conveniently situated, as organs by which the presence of 

 food, or it may be of enemies, is mainly perceived. The pos- 

 terior or tail end bears in this animal a powerful propeller in 

 the form of a fin. We also see marked differences as regards 

 the dorsal and ventral surfaces, having an obvious relation to 

 the needs of the animal. These differences are partly of form 

 and partly of colour. Nothing need here be said on the former 

 head, but as to the latter we note that the dorsal surface is dark 

 while the ventral is pale, an arrangement which, as will be fully 

 explained elsewhere, makes the animal match its surroundings 

 much better than would otherwise be the case. Bilateral sym- 

 metry, therefore, is essentially a distinction between right and left, 

 anterior and posterior, dorsal and ventral. It may be taken as an 



