240 SUBSTITUTE FOR THE USUAL GRATING. 



stated, that this grating is formed of a strong descrii^- 

 tion of silk, which the animal has the power of 

 spinning, and which assumes, although under water, 

 the necessary degree of consistence. In one of the 

 cases in my possession, it is formed of a mass of 

 minute portions of vegetable matter, so thick as 

 almost to exclude water ; and two holes are formed 

 at the sides of the case, close to the extremity, for 

 the ingress of the fluid. In another, some small 

 bivalve shells {Cyclas cornea) are agglutinated 

 together : and as their convexity leaves some vacant 

 spaces between each shell, the object of the grating 

 is attained by a dilFerent procedure. It would seem, 

 therefore, that the larvae, although endued with the 

 power of forming a silken net-work, avoid the trouble 

 of doing so, where the abundance of suitable mate- 

 rials of a different description, renders such a labour 

 unnecessary. They ajjpear to have the power of 

 secreting the silken substance for some time be- 

 fore the period of their change into pupae, if, in- 

 deed, they do not possess it from the first. This 

 opinion was formed on one occasion, when I had a 

 few caddis- worms in a glass of water, for the pur- 

 pose of observing their movements. One of large 

 dimensions projected his body a considerable way 

 from his covering, untU it touched the case of a 

 smaller caddis-worm in its vicinity, and after a 



