12 COMPARATIVE ANATOMY 



mental points of structure; they belong in fact to the same 

 style of architecture. 



Now, consider the lobster. To begin with, it has a hard 

 outside armour (often but erroneously called its shell), but 

 neither true hairs nor scales. Break open the armour, and 

 you find no bones inside ; no skull, no vertebral column. 

 The foremost end of the lobster may be called its head, 

 but how different to the head of the dog or fish ! In the 

 first place it is fused with, and can scarcely be distinguished 

 from, the breast or thoracic region. You look in vain for a 

 nose, an olfactory organ. There are two pairs of long jointed 

 feelers borne in front of the head region, and these have 

 numerous bristle-like structures, some of which are considered 

 to serve as olfactory organs, but how different are they from 

 the similarly named organs in dog and fish ! The " ear " or 

 auditory organ of the lobster does not lie behind the eyes, but 

 is placed on the basal joint of the first pair of feelers, and is 

 not at all like our ear. The eyes of the lobster have no eye- 

 ball, are placed on a pair of moveable stalks, and each is com- 

 posed of a number of minute facets, very strikingly different 

 from the arrangement of the dog's or fish's eye. The jaws of 

 the lobster do not work up and down, but from side to side ; 

 there are several pairs of them, and a little study convinces us 

 that they are nothing more than so many modified pairs of 

 limbs. The lobster walks and swims. It walks by four pairs 

 of jointed legs, in front of which a quite similar pair of limbs 

 is modified to form the great nippers or claws. The posterior 

 region of its body is composed of hard rings, which move on 

 one another. On the under surface of these rings are as many 

 pairs of little limbs used in swimming, the last pair being 

 large, expanded, and forming with the last joint of the body, a 

 posterior expansion used in vigorous swimming movements. 

 Lastly, we find that there is no skull with its enclosed brain in 

 the lobster, and no vertebral column and spinal cord, There 

 is a little nervous mass in front of the mouth, but posteriorly 

 the nerve cord is on the lower instead of on the upper side of 

 the body. The lobster does breathe water by structures called 

 gills, but these are quite unlike the gills of the fish, and are 

 simply bottle-brush-like appendages borne on the body-wall 

 or on some of the legs, and covered over by a fold of the 

 thoracic region of the body-wall. In so far as the jaws, the gills, 



