THE TUBE-WITHI N-A-TUBE BODY PLAN 



195 



Research on the nervous system of the 

 earthworm has revealed that it possesses 

 the components of reflex arcs very similar 

 to those of man ( Fig. 10-16). Impulses come 

 in from the external world through the 

 sense organs on afferent, or sensory, fibers, 

 pass to association neurons, and from these 

 to efferent, or motor, fibers, which run out 

 to muscles or glands. Thus in a form as low 

 as the earthworm, there is an intricate nerv- 

 ous mechanism which enables the animal to 

 carry out complex operations. 



It is of interest to biologists as well as 

 psychologists to know when the nervous 

 systems of animals become complex enough 

 to permit the storage of response patterns 

 which we call memory. In other words, how 

 far up the tree of animal life must we go 

 to find animals that can profit by ex- 

 perience, or learn? One experimentalist 

 (Swartz, 1929) found that an earthworm 

 which at first would enter either branch of a 

 Y-shaped tube could learn, after several 

 hundred trials, to avoid one branch of the 

 tube if an electrode were placed in it and 

 the worm received a shock each time it 

 entered that branch. This is perhaps the 

 first animal so far considered that can profit 

 by past experience. 



There are no obvious sense organs pres- 

 ent on the earthworm's body but micro- 

 scopic examination of the epidermis reveals 

 several kinds of sensory cells scattered 

 among the ordinary epithelial cells which 

 connect directly to the nervous system and 

 function as sense organs (Fig. 10-16). Such 

 cells are located in the parts of the body 

 that are most likely to come in contact with 

 the environment: the prostomium, the por- 

 tion of largest diameter in each segment, 

 and the mouth cavity. Some of these cells 

 are specialized for light reception, while 

 others respond to chemical and tactile 

 stimulation. 



The earthworm responds readily to light. 

 If removed from the burrow, the worm 

 becomes very active and immediately at- 

 tempts to get away from the bright light 



and crawl into a crevice or burrow. If 

 sought for at night with a flashlight, as is 

 the habit of those who search for the so- 

 called "night crawlers," the moment the 

 light strikes, the worm retracts into its 

 burrow with almost lightning-like speed, so 

 fast that one must be very agile to catch it. 

 It appears, then, that light is readily per- 

 ceived and interpreted. 



If an earthworm is experimentally placed 

 near any volatile chemical, such as acetic 

 acid, it responds violently and moves in the 

 opposite direction. Likewise, it moves to- 

 ward bits of meat or decaying vegetation, 

 which, of course, are its food. In this case 

 the sense organs appear to be located in the 

 mouth cavity. 



Since the earthworm is dependent on 

 being able to find its way around under- 

 ground in completely dark passages, it 

 must rely on the sense of touch perhaps 

 more than any other. The tactile end organs 

 ( Fig. 10-16) are bundles of modified epithe- 

 lial cells with tiny protruding hair-like 

 bodies that are in contact with anything 

 that touches the body wall. They probably 

 also function as a sort of hearing device, 

 since any vibration on the earth near the 

 burrow, as in the case of footsteps, causes 

 the worm to respond readily. They do not 

 respond to air-borne vibrations which af- 

 fect our ears. 



The earthworm is sensitive to tempera- 

 ture; it avoids cold and hot areas and seeks 

 out moderate temperatures. In its natural 

 environment it lies near the surface in its 

 vertical burrow with the "head" uppermost 

 when the temperature is moderate, reced- 

 ing if it is too cold or too hot. In winter 

 it burrows down below the frost line and 

 remains inactive throughout the cold sea- 

 son; it does likewise when the soil becomes 

 dry during a drought. It cannot tolerate 

 desiccation and therefore responds posi- 

 tively toward moisture, but only to a certain 

 point, since it cannot withstand immersion 

 in water for a long period of time. After a 

 rain it is common to see on sidewalks earth- 



