78 INVERTEBRATE ZOOLOGY 



instances altered by the fusion of two or more successive ganglia 

 to form a single mass. This tendency is especially pronounced in 

 the anterior region of the body where a single prominent ganglion, 

 the brain, is located. 



In many of the worms, two or more lateral nerve cords pass 

 through the body, while in some of the annelids and in the arthro- 

 pods these have been fused to form a single chain of ganglia 

 ventral to the digestive tract. An examination of a cross-section 

 of such a nerve cord, however, usually furnishes evidences of its 

 double nature. 



That portion of the nervous system which contains the ganglia 

 and consequently is the controlling center for directing the activ- 

 ity of the organism is designated as the central nervous system, 

 while from it fibers and bundles of fibers pass to the various parts 

 of the body as a peripheral nervous system. Largely through the 

 latter are the sense organs brought, into relationship with the cen- 

 tral nervous system. A sympathetic system in some of the higher 

 arthropods controls the activity of some of the internal organs. 



As has been pointed out, there are numerous modifications of 

 epithelial tissues for receiving stimuli and transmitting nervous 

 impulses to the central nervous system of the invertebrates. 

 Concerning many of these sensory organs our knowledge is but 

 fragmentary, for they are organs which have no counterpart in 

 the human body and consequently direct knowledge of their 

 functions is beyond the scope of human experience. In some 

 instances, their functions may be inferred from the location and 

 the mechanics of their structure and through the reactions of the 

 organism when the organ in question is stimulated. 



Tactile organs are very commonly developed as hairs or bristles 

 from the surfaces of epithelial cells, or, as in the case of bristles of 

 arthropods, they may be formed as outgrowths of specialized cells. 

 Tactile organs may be scattered fairly uniformly over the 

 surface of the animal, but sometimes specific organs are especially 

 adapted for receiving tactile stimuli, as, for example, the tentacles 

 of coelenterates, worms, and snails and the antennae and palpi 

 of insects and crustaceans. In many such instances, other sen- 

 sory organs are associated with those of touch. Thus the 

 olfactory organs of insects are commonly found on the antennae 

 along with other sensory organs of uncertain function. In some 

 instances, the antennae are provided with hairs which seem to be 

 associated with an auditory function. 



