26 INTRODUCTION. 



but not to the same extent as those of man, owing to the 

 low degree of their intelligence. 



The organs of the anterior and posterior extremities 

 connected with the trunk or spinal column in the higher 

 vertebrata, are gradually absorbed in the lower, until at 

 length in the serpent tribe, these locomotive appendages are 

 suppressed altogether, and the body of the animal consists 

 of little else but the spinal column itself, which is very 

 long and extremely flexible, owing to the immense num- 

 ber of vertebra, and their connection with each other by a 

 ball and socket joint. The perceptions of the animal are 

 now obtuse, and all its movements sluggish, — a mere 

 trailing of the body along the ground. 



The same gradual simplification of the nervo-muscular ap- 

 paratus, may be traced throughout the descending series of 

 invertebrated animals. Insects may be truly regarded as the 

 most highly developed of the invertebrata. In them the ani- 

 mal functions are decidedly more developed than the vegeta- 

 tive. Their rapidity of motion, and extraordinary display of 

 intelligence, entitle them to this position. The tegumentary 

 skeleton of insects is composed of a number of movable 

 pieces articulated to each other, and is of a horny texture. 

 This integument becomes progressively hardened, and the 

 pieces fewer in number, and more consolidated in the 

 different orders of the Crustacea, so that the movements are 

 necessarily much more restricted and confined. In the 

 testaceous mollusoa, the integument is finally reduced to a 

 pair of valves, and the muscular movements of the animal 

 are of the simplest character. Most of the bivalve muscles, 

 such as the cardium, move along by means of a fleshy organ 

 called a foot. The movements of the oyster are restricted 

 to the single act of opening and closing its shell, and those 

 of serpulse and limpets, to the alternate protrusion and 



