STRUCTURE OF ORGANISMS 473 



2. Ganglionic Synaptic Nervous System.— 'Nervous systems of this 

 type possess neurons the cell bodies of which are in ganglia. In the 

 mollusks these gangha are more or less scattered, but in the annelids 

 and arthropods they are arranged segmentally along a double ventral 

 nerve cord (Figs. 165 and 188). 



3. Tubular Synaptic Nervous System.— This type of nervous system, 

 characterized by a dorsal central nervous system which is tubular in 

 shape, is characteristic of chordates and has been described in connection 

 with them (Sec. 336). 



In the tracing of the nervous systems of the various phyla two 

 phenomena have been noted, centralization and cephahzation. These 

 have been referred to in Chap. LXI. 



As epithelial cells become modified to form receptors, taking the 

 place of receptor neurons which in lower types lie upon the surface of 

 the body, they come to be specialized so that each group receives only 

 a certain kind of stimulus. A great variety of accessory structures are 

 added to the organs which increase their effectiveness. The sense organs 

 of vertebrates, especially such organs as the eye and ear, become in this 

 way exceedingly complex. 



Those receptors which receive stimuli from without are known as 

 exteroceptors. Such receptors are those associated with the chemical 

 senses of taste and smell; those receiving contact stimuli, giving rise to 

 sensations of touch, pressure, and pain; those receiving temperature 

 stimuli, giving rise to sensations of heat and cold; and those stimulated 

 by sound vibrations and by light. There are others which are known as 

 interoceptors, stimulated by conditions within the digestive system and 

 giving rise to sensations such as hunger, nausea, and visceral pain. 

 There is also a third group, known as proprioceptors, which are stimulated 

 by vital processes within the organism itself. Included in these are the 

 pressure receptors of muscles, tendons, hgaments, and other internal 

 organs, pain spots, and organs of position which give to the animal a 

 sense of equilibrium. 



483. Convergence and Divergence, — To the field of morphology 

 belong the phenomena of analogy and homology, which were early 

 defined. Homology may exist between organs or parts metamerically 

 arranged in the same body ; between those on the opposite sides of a 

 bilaterally symmetrical animal; between the antimeres of a radially 

 symmetrical animal; or between corresponding organs and parts of dif- 

 ferent individuals. Analogy involves no correspondence in manner of 

 origin and only such correspondence in position as is mechanically neces- 

 sary to the performance of a function. An illustration of the latter is 

 seen in the case of wings, which have to possess a certain position with 

 reference to the center of gravity of the organism, or of a locomotor fin, 

 which has to be at the posterior end of the body. Phenomena which are, 



