84 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



sciousness as a specific sensation. To these end-cells there are often 

 added cuticular structures, whose function is to communicate the 

 external movement to the nervous substance (retinal rods). 



The special sensations have quite gradually been developed from 

 the general sensations (comfort, discomfort, pleasure, pain), i.e., 

 nerves of special sense have been derived from sensory nerves which 

 have acquired a special form of peripheral termination, and so 

 become accessible to a special stimulus with which the special 

 sensation is always associated. But it is not till a higher stage of 

 development is reached that the sense-perceptions can be compared 

 according to the nature of the sensations with those of our own body. 

 We can estimate the sense energies of the lower animals exceedingly 



vaguely, and only by the 

 insufficient method of com- 

 paring them with our own 

 sensations; and it is certain 

 that among the lower ani- 

 mals there are many forms 

 of sensation of which we, 

 in consequence of the spe- 

 cialised nature of our own 

 senses, can have no concep- 

 tion. 



Probably of all the 

 senses, that of touch is the 

 most widely distributed, 

 and with this we certainly 

 often see a number of 

 special sensations united. 

 It is generally distributed 



over the whole surface of the body ; frequently, however, it is con- 

 centrated on processes and appendages of it. Probably the tentacular 

 appendages of the Ccelenterata and Echinodermata have this signifi- 

 cance. In the Bilateralia with a differentiated head there are 

 contractile or stiff segmented processes on the head, the antennce or 

 feelers which in the worms are repeated as paired cirri on every 

 segment of the body. It is often possible to trace special nerves 

 to the skin and to find touch organs containing their endings. In 

 the Arthropoda the ganglionic end-swelling of a tactile nerve usually 

 lies beneath a cuticular appendage, such as a bristle, which transmits 

 the mechanical pressure on its point to the nerve (fig. 81). 



FIG. 81. Nerves with ganglion cells (G) beneath a 

 tactile bristle (TV) from the skin of Corethra larva. 



