Shelter Building 257 



forward to distend the body there, thus widening the 

 burrow. x\nd if anyone would see how such a larva gets 

 through a narrow space when the walls cannot be 

 pushed farther apart, let him wet his hand and close the 

 larva in its palm ; the larva will quickly slip out between 

 the fingers of the tightly closed hand; and when half 

 way out it will present a strikingly dumb-bell-shaped 

 outline. Here, again, we see the advantage of its 

 almost fluid interior. 



This adjustability of body, is of course, not peculiar to 

 soft bodied insect larvae ; it is seen in leeches and slugs 

 and many worms. 



The mussel's mode of burrowing is not essentially 

 different from that above described. The slender 

 hollow foot is pushed forward into the sand, and then 

 distended by blood forced into it from the rear. When 

 sufficiently distended to hold securely by pressure 

 against the sand, a strong pull drags the heavy shell 

 forward. 



Ill 



Shelter building — Some animals produce adhesive 

 secretions that harden on contact with the water. 

 Thus, these are able to bind loose objects together into 

 shelters more suitable for their residence than any that 

 nature furnishes ready made. The habit of shelter 

 building has sprung up in many groups; in such 

 protozoans as Difflugia (see fig. 69 c on p. 39) ; in such 

 worms as Dero (see fig. 82 on p. 174) ; in such rotifers 

 as Melicerta (see fig. 86 on p. 178) ; in such caterpillars 

 as Hydrocampa (see fig. 127 on p. 219); in nearly all 

 midges, as Chironomus (see figure 134 on p. 226) and 

 Tanytarsus (see fig. 134 on p. 226); and especially in 

 the caddis-worms, all of which construct shelters of some 

 sort and most of which build portable cases. The 

 extraordinary prevalence in all fresh waters of such forms 



