34 



Net making. 



Drawing in nets. 



Advantages of the 

 net. 



To build its net the larva proceeds as follows : running up one of 

 the arms for some distance, it swings across to the next arm carrying 

 with it a thread of silk, then quickly back again, at the same time 

 retreating somewhat into its case ; this zigzag movement is repeated two 

 or three times until the base of the arms is reached, when the whole 

 process may be repeated over again, until a sufficient number of threads 

 have been stretched across to make a rude net-work which, whatever 

 its workmanship compared with that of a spider, is at any rate good 

 enough for its purpose, and effectually stops all objects floating by. In 

 the case of larva No. i this has only to be done twice, but in C. pnsio 

 from four to seven times according to the number of arms present. 

 From time to time the larva pulls down the net between two arms, 

 using the labrum and thoracic feet to collect the particles together into 

 a compact mass, which may then be used for further building operations, 

 or may be pressed into the mouth to be devoured. 



Both these larvae seem never to extend the full length of their 

 bodies from their tubes, but always retain at least one of the segments 

 bearing the bifurcating hairs, that is to say not less than the four 

 posterior segments. 



That the net is useful to C. pusio and to larva No. i is obvious 

 from what has been already said, but it is interesting to discover what 

 are the advantages gained by these larvae over the net-less species, of 

 which there are at least three, all about the same size, inhabiting cases 

 without arms and nets, and living together in the same situations. If 

 these can procure all they require without a net to catch it, what can 

 it benefit the net-builders to go to so much pains ? Two reasons, I 

 think, may be given. In the first place the food eaten by the net- 

 builders must be of a much finer quality than that of the others, for 

 the nets would catch all the light flocculent matter, mostly organic, 

 which remains suspended in the water, while the other larvae would have 

 to feed on a much coarser pabulum containing much sand and grit. In 

 the second place, the net-builders occupy a more superficial stratum 

 of water containing fewer creatures ; being thus elevated from the 

 stones, and suspended freely from the moss, they would escape many 



