EVOLUTION AXD THE AFTER-LIFE. 



49 



conimuiiicating nerves to the ganglion; there a " motor impulse " is 

 generated, which again is translerrecl along the communicating lines 

 from the ganglion to the muscular envelope, causing it to contract 

 and eject the water which it contained, together with the offending 

 substance. This is all the action the creature is capable of, and this 

 it repeats whenever and wherever an irritation is applied. It is the 

 simplest action, so far as we know, requiring a nervous system for its 

 accomplishment, and has received the name of reflex action. 



Without following each shade of improvement in the nervous or- 

 ganization of the mollusks, or soft-bodied animals, it may suffice to 

 say that the same general arrangement holds throughout all the lower 

 membei's of the series, new ganglia being added to meet the needs of 

 a more complex organization until, in the highest members, as, for 

 instance, in snails and the cuttle-fish, important changes are found to 

 have occurred. Instead of the headless and irregular masses which 

 have constituted the bodies up to this point, we now find an animal 

 comparatively symmetrical in form, w'ith a distinct head, and im- 

 l^ex-fectly-developed organs of special sense. Here, then, occurs the 

 division between the two great classes of animals known as cephalous 

 and acephalous; all the lower species of mollusks, the ascidians, mus- 

 sels, oysters, and the like, belong to the acephalous or headless class, 

 while with the highest species, snails, the nautilus, and the cuttle-fish, 

 commences the other great class known as cephalous animals, or those 

 having distinct heads. 



The advance in the nervous sj'Stem is correspondingly great ; 

 instead of the irregularly-situated ganglia hitherto met with, we find 

 them arranged in pairs, to serve the purposes of the more symmetrical 

 body ; but a still more marked and important advance is found in the 

 fact that each organ of special sense (sight, hearing, etc.) is furnished 

 with a separate nerve-centre or ganglion ; all of which, being brought 

 together in the head of the animal, constitute what is known as the 

 cephalic ganglia, or sensorium, an incipient brain. 



The function of this nervous system is not merely to resj^ond to 

 irritation applied directly to the body, but also to respond to certain 

 stimuli, sucli as that afforded by sight and hearing, received at the 

 sensorium through the organs of special sense, and thence transferred 

 to the difierent ganglia. This new stimulus is called sensation, and it 

 is one to which tlie ganglionic system responds with almost the same 

 alacrity that it does to direct contact. 



Leaving now the grand division of mollusks, we ascend to another 

 division in the animal w^orld, namely, the Articulates. It embraces 

 such marine animals as the crab, lobster, and crayfish ; and on the 

 land, worms, centipedes, and all the numerous tribes of insects. The 

 characteristic of the Avhole division is, that the body is made up of 

 rings or segments, joined and moving one upon the other, and hence 

 the name articulates. 



TO!.. VII. 



