INTEGRATION OF ACTIVITIES 



141 



projections, usually branching (Fig. 115) and joining one another to 

 form a network. The spread of these cells through the ectoderm is 

 fairly uniform, though they are slightly more abundant at the foot and 

 among the bases of the tentacles and around the mouth. Hydra's close 

 relatives, the jelly fishes, have a ring of nerve cells around the edge of 

 their cuplike bodies, with a loose network over the remainder. 



Animals successively higher than the jellyfishes show a progressive 

 tendency to collect their nerve cells into masses or strands. In the 

 flatworms there is a mass of them, which may be called a ganglion, in 

 the anterior region (Fig. 116), and from this mass two long strands or 

 cords pass back on either side of the body. From both the ganglion 



Fig. 115. — Nervous mechanism of Hydra. The long fibrils in the background are the 

 contractile parts of neuromuscular cells lying in the mesogloea. {From Schneider.) 



and the cords slender threads called nerves extend to all parts of the 

 organism. 



Invertebrate animals above the flatworms generally have two longi- 

 tudinal nerve cords, but these are usually joined into a single cord in 

 which the two components are still easily recognizable. In the earth- 

 worm (Fig. 116) these cords separate in the anterior region, pass upward 

 around the digestive tract in the form of a collar, and become enlarged 

 above the tract to form the bilobed brain. The rest of the double cord 

 in the earthworm is swollen into a moderate ganglion in each segment, 

 and from this ganglion two pairs of nerves emerge. The ganglia of the 

 main nerve cords are much larger in the crayfish (Fig. 116) and its allies, 

 with the larger ganglia located toward the front. 



The tendency to mass the nerve tissue in a head region is carried 

 much farther in vertebrate animals. In them there is always a dis- 

 tinctly enlarged brain. In the frog it is moderately larger than the cord 

 behind it, which in the vertebrates is known as the spinal cord. The 



