154 COMPARATIVE PHYSIOLOGY 



by Eimer, have been amplified and extended by those of Mayer 

 (1905), Loeb (1906), Bethe (1909), Harvey (1912), and others. 

 These workers have conclusively shown in various ways that 

 conduction takes place through the nerve-net and not through 

 the muscle. This is quite easy to show in the jelly-fish 

 Rhizostoma (Bethe), where the muscle of the bell and nerve- 

 net are not coextensive, inasmuch as the " sphincter " is 

 composed not of a continuous band of muscle as in Aurelia, 

 the form with which Romanes worked, but of sixteen separate 

 areas with intervening non-muscular tissue across which the 

 nerve-net extends. 



The Synaptic Nervous System. — In contrast with the nerve- 

 net of the ccelenterate and the generaUsed modes of re- 

 sponse associated therewith, we now turn to the synaptic 

 nervous system of echinoderms, annelids, molluscs, arthro- 

 pods, and vertebrates. The unit of response in these animals 

 is essentially locaUsed ; the stimulation of any group of re- 

 ceptors calls forth response in a strictly limited number of 

 effectors. To illustrate more concretely the conception of 

 the reflex the following observations of Bethe (1897) on the 

 behaviour of the shore crab (Carcinus) will serve. 



1. When the eye is blackened to exclude photic stimuli, 

 gentle mechanical stimulation causes the eye which is touched 

 to be drawn under the carapace ; both of the antennules are 

 simultaneously withdrawn. When the same eye is subjected 

 to a more powerful mechanical stimulus, in addition to the 

 withdrawal of the antennules and the eye itself the second 

 antenna of the same side is withdrawn. The opposite eye 

 and corresponding second antenna are not affected. 



2. When the covered eye is subjected to electrical stimula- 

 tion, the walking legs of that side are brought into such a 

 position that the body tends to be tilted forwards from the 

 ground at an angle of 45°. If both eyes are stimulated the 

 body tilts upward symmetrically, both chelae being extended. 

 Only one chela is involved as a result of unilateral stimulation 

 of the eye. 



3. Weak mechanical stimulation of the second antenna 

 provokes first the withdrawal of the antennule of the same 



