PHYLUM ANNELroA. SEGMENTED WORMS 



183 



tida; this positive reaction to faint light may 

 account for the emergence of the worms 

 from their burrows at night. It is an inter- 

 esting fact that although the worms react 

 negatively to sunlight, they respond posi- 

 tively to red light and may be collected at 

 night with the use of such a light. 



Physiologic state 



From the foregoing account, it might be 

 inferred that only external stimuli are fac- 

 tors in the behavior of the earthworm. This, 

 however, is not the case, since the physio- 

 logic condition, which depends largely upon 

 previous stimulation, determines the char- 

 acter of the response. Different physiologic 

 states may be recognized, ranging from a 

 state of rest in which slight stimuli are not 

 effective, to a state of great excitement 

 caused by long-continued and intense stimu- 

 lation, in which condition, slight stimuli 

 cause violent responses. By physiologic states 

 we mean the varying internal physiologic 

 conditions of the organism as distinguished 

 from permanent anatomic conditions. Such 

 different internal physiologic conditions can 

 be inferred from the behavior of the animal. 



Learning in earthworms 



Whether or not learning occurs in proto- 

 zoans, or in such simple metazoans as 

 sponges and hydras, is uncertain. But at 

 the stage in evolution represented by the 

 earthworm, experiments indicate that this 

 animal is capable of what psychologists call 

 "latent memory," or the storing of impres- 

 sions until a later time when they may be 

 useful. 



In one experiment, worms could escape 

 from a lighted chamber by entering the bot- 

 tom of a branched passageway constructed 

 of glass tubing in the form of a "T." If 

 the worms turned to the right at the top of 

 the "T," they entered a dark moist cham- 

 ber filled with damp earth and moss, a 

 favorable environment for an earthworm. 

 If they turned left, they encountered an 

 electric shock. In the eariy trials, they turned 



to the left as often as to the right. At the 

 end of 20 days, they turned to the left only 

 5 times out of 20, and at the end of 40 days 

 they were turning left only once out of 20 

 trials. 



OTHER ANNELIDA 



Annelids differ from the other groups of 

 "worms" in the following respects: (1) the 

 body is divided into a linear series of similar 

 segments, often visible externally because of 

 grooves that encircle the body, and inter- 

 nally because of partitions called septa; (2) 

 the body cavity between the digestive tract 

 and body wall is a true coelom; (3) the 

 mouth opens in the first segment and is 

 overhung by the prostomium; (4) the nerv- 

 ous system consists of a preoral ganglion, the 

 "brain," often bilobed, and a pair of ventral 

 nerve cords, typically with a pair of ganglia 

 in each segment; (5) usually, a nonchitinous 

 cuticle on the surface of the body; chitinized 

 bristles or setae are present. 



The sandworm 



Neanthes virens is a common polychaete 

 that lives in burrows in the sand or mud of 

 the seashore at tide level. By day it rests 

 in its burrow, but at night it extends its 

 body in search of food or may leave the bur- 

 row entirely. 



The body is flattened dorsoventrally and 

 may reach a length of 18 inches or more, 

 with 100 to 200 or more segments. The 

 head is well developed. Above the mouth is 

 the prostomium (Fig. 99) which bears a 

 pair of terminal tentacles, 2 pairs of simple 

 eyes, and, on either side, a thick palp. The 

 first segment is the peristomium; from each 

 side of this arise 4 tentacles. Small animals 

 are captured by a pair of strong chitinous 

 jaws which are everted with part of the 

 pharynx when Neanthes is feeding. Behind 

 the head are a variable number of segments 

 each bearing a fleshy outgrowth on either 

 side, the parapodium (Fig. 99). 



