46 THE FLEA [OH. 



body under the digestive canal of the flea, whereas in 

 the vertebrate it lies along the back and above the 

 digestive canal. The dorsal spinal cord of the verte- 

 brate is then a ventral nervous cord in a flea. 



The sensory nerves, which transmit sensations 

 from different sense-organs, and the motor nerves, 

 which send stimuli to the muscles, take their origin 

 from other ganglia besides the ganglion above the 

 gullet. In bees and some other insects it has been 

 shown that the nerves from the palpi and mouth- 

 parts go to the next ganglion which is beneath the 

 gullet. The same is probably the case with fleas ; so 

 when we speak of the brain of a flea we must re- 

 member that it has a relative rather than an absolute 

 claim to that title. A flea has really many brains. 



In certain blind insects, where the eyes are 

 wanting, parts of the brain are completely atrophied. 

 Whether this is so in the blind species of fleas does 

 not seem to have been investigated. 



We pass now from the central nervous system to 

 the sense-organs of the flea. The chief are the eyes, 

 the antennae and the pygidium. In regard to the 

 eyes nothing more need be said. The antennae are 

 probably far more important organs to a flea than its 

 eyes ; but inasmuch as they are at ordinary times 

 concealed in a groove they are not very conspicuous 

 (Fig. 5). The first tolerably accurate plate of a flea 

 by a naturalist will be found in Hooke's Micrographia 



