CHAPTER VII. 



GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS. 



In the arachnids, we may recognize three main groups of sense organs; 

 primitive segmental, special cutaneous, and general cutaneous. 



a. The primitive segmental sense organs include the median and the lateral 

 eyes, the olfactory and the auditory organs. With the exception of the last'named, 

 they are so highly developed and have been so long established that they and 

 their nerve centers constitute the very foundations of the forehead and forebrain. 

 h. The special cutaneous sense organs include the gustatory buds, slime buds, 

 and other chemotactic, or tactile organs that have v^^ell defined nerves, ganglia, 

 and central terminals, and that are located in definite fields, or areas, such as 

 the coxal spurs, chilaria, pectines, iiabellum, etc. They attain their highest 

 development in the midbrain and hindbrain regions, c. The general cutaneous 

 sense organs may be of the same nature as the special cutaneous, but they are 

 irregularly distributed, and without separate, well defined nerves or ganglia; 

 we also include in this category temperature organs and free nerve endings. 

 They are supplied mainly by the subdermal plexus formed from the terminal 

 branches of the neural, but especially of the h^mal nerves. These nerves may 

 arise from all the main divisions of the neuron except the forebrain. 



I. General Cutaneous Sense Organs. 



Temperature Organs. — Reaction to changes in temperature in the lower 

 animals is probably much more delicate and more generally exercised than has 

 been suspected. The temperature sense is not dependent on the location of its 

 organs in a particular part of the body, for changes of temperature are diffusely 

 distributed, and are not likely to affect the animal at one point more than another. 

 An effective response results in the transfer of the whole body to surroundings of 

 a different temperature; thus the temperature organs primarily control them ove- 

 ments of the animal as a whole, or its migrations, or distribution in space. 



One would hardly suspect that such a heavily armored animal as Limulus 

 would be very sensitive to changes in temperature, yet that such is the case may be 

 easily demonstrated. When placed on Its back and allowed to become perfectly 

 quiet, it may be fanned, or the surface of the carapace, or the gills, or the legs, 

 may be touched with an object the same temperature as the air, without causing 

 any reflexes; but the instant any of these parts are touched ever so gently with the 

 finger, or if water a little warmer or colder than the surrounding air falls on them, 



no 



