396 ElOHABD EVANS. 



the same larva fixes by the side the solid posterior j)art of the 

 larva topples over on to one side, and the larval cavity is 

 obliterated by the coming together of the opposite sides. In 

 this case the larva flattens ont as before, but owing to the fact 

 that the greater number of cells are situated on one side, 

 it lacks the radial symmetry characteristic of the larva that 

 fixes by the anterior pole, and presents a temporary variation 

 of form, which, however, is soon lost. On the other hand, it is 

 very doubtful if the larval cavity is ever completely obliterated 

 at the fixation of type C, whether the larva settles down by the 

 anterior pole or by the side. In type C either the chambers are 

 already formed — their individual elements being adorned with 

 collars and flagella — or they will be formed soon after fixation. 

 In either case the young sponge can ill spare, so to speak, the 

 time necessary for the destruction of the larval cavity, and the 

 subsequent new formation of the inhalant and exhalant systems 

 of canals. Besides, it has been shown above that in some 

 larvae the chambers, being fully formed, already open into small 

 spaces and lacunar canals, which represent the beginnings of the 

 exhalant system, and that these canals open in turn into the 

 larval cavity, at least in some cases. Individuals possessing 

 the above characters have been seen soon after fixation to 

 possess so large a cavity, evidently derived from that larva, 

 that it is impossible to believe that the larvae in question 

 would have lost it had they been allowed to develop to 

 maturity. The fact that the flagellated chambers and the 

 finer portions of the exhalant system have been developed 

 already, and that nearly all the food material stored in the egg 

 cell has been used up, render it absolutely necessary that the 

 canals along which the current carrying food material for the 

 young sponge is to pass should be in. working order as soon as 

 possible Taking these facts into consideration, it may be 

 fairly concluded that the larval cavity is not usually obliterated 

 after the fixation of type C, but becomes a portion of the 

 exhalant canal system and gastral cavity. 



The Metamorphosis. — In describing the metamorphosis, 

 which is a process involving the reversion of the layers that 



