SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 191 



Nerve net. In the lower Metazoa special cells have been differentiated 

 for the reception and conduction of stimuli. In Hydra these sensory cells 

 are located in the body wall, and in the inner layer of cells surrounding 

 the coelenteron branches extend between the sensory cells and from 

 them to the contractile processes of the body-wall cells. Thus a network 

 of sensory cells and cell processes is formed between the two layers of the 

 body. (See enlarged detail in Fig. 13.5.) Because of the lack of concentra- 

 tion of sensory cells, the nerve net conducts diffusely; chemical, me- 

 chanical, and light stimuli affect it most strongly. 



Simple nerve cords and formation of a head end. In the inter- 

 mediate Metazoa there first appears a concentration of nervous tissue 

 and sense organs in the anterior end of the body — the formation of a 

 head end (cephalization). This is first seen in the flat worms, elongated 

 ribbonlike animals, in which the anterior end of the body is specialized. 

 The head end of the flatworm is the part of the body most richly supplied 

 with sense organs (Fig. 13.6, C), and sensitivity decreases toward the 

 posterior end. There is a massing of nerve cells at the anterior end to 

 form a cephalic ganglion, which dominates the nerve system of the animal 

 and which receives nerve impulses from the sensory organs of the head. 

 The head end thus comes largely to control behavior, and impulses from 

 the cephalic ganglion are transmitted through the two lateral nerve trunks 

 to all other parts of the body. 



The ventral nerve cord and its cephalic ganglion. In the segmented 

 worms (for example, the earthworm, Fig. 13.7) and the arthropods (for 

 example, an insect), there has been a marked advance in the degree of 

 cephalization of nerve tissue and sense organs, which reaches a maximum 

 in flies, wasps, and spiders. The enlarged and complicated cephalic 

 nerve mass may be termed a brain. Leading from the brain, which is 

 located above the esophagus or pharynx, two nerve strands encircle 

 the digestive tube and unite below into a secondary ganglion. From this 

 subesophageal ganglion a double nerve cord runs down the ventral sur- 

 face of the animal, swelling into small ganglia and giving off lateral 

 nerves in each of the body segments; it is thus somewhat chainlike or 

 ladderlike in appearance. As one proceeds from lower to higher groups 

 within the insect series, there is a tendency for this system to become 

 more and more highly concentrated in the anterior portions of the body. 

 The ganglia and nerve cords at the same time become larger and more 

 compact. Many of the insects with the largest and most concentrated 

 nervous systems (for example, ants, wasps, bees) also show the most 

 complex and highly adaptive behavior, even extending to social and 

 psychic phenomena. These facts suggest that efficient control of a com- 

 plicated organism requires a concentrated rather than a diffuse nervous 

 system. 



