43 



to make no difference how often the child's feet may be placed in 

 contact with the ground, the legs remain limp and do not respond to the 

 sensation of contact in the soles by muscular contractions pressing down- 

 wards. No sooner however is the standing impulse born than the child 

 stiffens his legs and presses downwards as soon as he feels the floor." 



When the child has acquired the habit of walking, the feeling 

 tone in the feet gradually diminishes until it becomes unconscious of 

 it altogether. So also with the case-dwellers, when they have acquired 

 the habit, the primary sensation becomes less important and consequently 

 the sense organ may degenerate without loss of the corresponding action. 



Some of the case-building Trichoptera have neither fleshy 

 prominences nor sensitive fringes to generate a retiring habit ; how then 

 are we to account for the instinct in these species ? I think it will be 

 found generally that where it is impossible to explain on sensational 

 grounds, mechanical reasons can be shown to exist to prevent the larva 

 from escaping from its case. (If not, we must be content to leave it 

 for a time among the pure instincts due to the innate mental constitution 

 of the animal, or better still confess our ignorance and own that we do 

 not know the cause.) 



Many species of Setodes and Hydrophilidae, of which two species 

 are found in the Yealm among the Chironomids, have no sensitive fringes, 

 but these larvae make cases of hardened silk with very narrow necks 

 so that the head and thorax can just pass out, but the abdomen, which 

 in these species is very large and fleshy, is quite unable to pass through 

 so small an opening however much the larva might desire it. 



Many of the Polychaete worms inhabit beautifully constructed tubes, 

 and some even make movable cases after the manner of the Trichoptera. 

 I shall therefore give two examples, one to illustrate the physchological 

 and the other the mechanical means by which the animal may be kept 

 from extending too far from its case. 



Hyalinoecia tubicola builds a nearly transparent horn-like case 8 cm. 

 long by 5 mm. wide, quite open at both ends, tapering and somewhat 

 curved. The worm carries on each segment a pair of parapodia ; each 



The retiring habit 

 in the Polychteta. 



Psychological 

 control. 



