Life of Plants and Animals 221 



more complex nervous system, occupying a dorsal 

 neural tube, is developed, the homologue of which is 

 wanting in the invertebrates. It consists of a spinal 

 cord, from which there proceed spinal nerves to the 

 'Voluntary muscles of the body. 



In the higher vertebrates a brain, consisting of the en- 

 larged part of the cord, the bulb or medulla oblongata, 

 the optic lobes, cerebellum, and cerebral hemispheres, 

 are added to the anterior end of the cord occupying 

 the skull. This part of the nervous system is found 

 to correspond in mass and complexity to the general 

 intelligence of the animal. Hence the cerebral hemi- 

 spheres are larger and more convoluted in the ox than 

 in the frog. In the fish the optic lobes are very large, 

 comparatively, showing inferior development of the 

 organ of intelligence, the cerebral hemispheres. A 

 comparison of the nervous system of different animals 

 shows a gradual concentration of the nervous matter 

 in the head, where most of the special sense-organs 

 also are developed. 



Sense-organs are modified cells at the peripheral 

 termination of nerves. Sense-organs of touch are 

 most numerous, the sense of touch being the most 

 general of all the senses. Even these tend to become 

 aggregated in those parts that are used chiefly as 

 tactile organs, such as antennae, the lips, tongue, 

 limbs, etc. The more special sense-organs, like the 

 eye and ear, seem to be formed by the union of several 

 simple organs. They are infoldings of the skin. 

 Eyes are at first scarcely more than pigment spots, 

 having the power of responding vaguely to the influ- 

 ence of light. Such primitive eyes can first be seen 

 in medusae, and are frequently found in worms scat- 

 tered over the back, as in planarians or flatworms. 



Some mollusks have well-developed eyes on the 

 edge of the mantle, others on antennae or on the head, 

 and the squid and its allies have highly developed 



