OF THE NERVOUS SYSTEM. 



375 



the other by means of commissures will be a repetition, or rather 

 a simultaneous production of similar movements in different parts 

 of the body. According to the character and intensity, therefore, 

 of the original stimulus, it will be followed by a response from 

 one, several, or all of the different parts of the animal frame. 



It will be seen also that there are two kinds of nervous filaments, 

 differing essentially in their functions. One set of these fibres run 

 from the sensitive surfaces to the ganglion, and convey the nervous 

 impression inward. These are called sensitive fibres. The other 

 set run from the ganglion to the muscles, and carry the nervous 

 impression outward. These are called motor fibres. 



In the starfish, where the body is composed of a repetition of simi- 

 lar parts arranged round a common centre, and where all the limbs 

 are precisely alike in structure, the several ganglia composing the 

 nervous system are also similar to each other, and act in the same 

 way. But in animals which are constructed upon a different plan, 

 and whose bodies are composed of distinct organs, situated in dif- 

 ferent regions, we find that the nervous ganglia, presiding over 

 the function of these organs, present a corresponding degree of 

 dissimilarity. 



In Aplysia, for example, which belongs to the type of mollusca. 

 or soft-bodied animals, the digestive apparatus consists of a mouth, 

 an ossophagus, a triple stomach, and a some- 

 what convoluted intestine. The liver is 

 large, and placed on one side of the body, 

 while the gills, in the form of vascular 

 laminae, occupy the opposite side. There 

 are both testicles and ovaries in the same 

 animal, the male and female functions co- 

 existing, as in many other invertebrate 

 species. All the organs, furthermore, are 

 here arranged without any reference to a 

 regular or symmetrical plan. The body is 

 covered with a muscular mantle, which ex- 

 pands at the ventral surface into a tolerably 

 well-developed " foot," or organ of locomo- 

 tion, by which the animal is enabled to 

 change its position and move from one NERVOUS SYSTEM OF 



1 TJ. ^i API.YSIA. 1. Digestive or 



locality tO another. oesophageal ganglion. 2. Cere- 



The nervous system of this animal is con- bral ganglion, s, s. Pedai or 



, , j , . , locomotory ganglia. 4. Kespi- 



structed upon a plan corresponding with ra tu y gaugiiuu. 



Fig. 124. 



