394 RICHARD EVANS. 



fore be fairly concluded that type D has been produced directly 

 from type A. 



From these considerations it seems that two divergent lines 

 of development can be distinguished in the larva of Spongilla, 

 of which the two culminating points are represented by types 

 C and D^ type B being an intermediate stage between types A 

 and C. Type C is not necessarily older than type D, or vice 

 versa, but both of them have attained that stage at which 

 they usually fix themselves. The differences exhibited by 

 these larvae are in no way more striking than those which will 

 be found in the newly fixed stages, as the results of these 

 variations. In fact, the structure of the larvae seems to cast 

 its shadow, as it were, over the whole period of metamor- 

 phosis, even up to the appearance of the young sponge. 



B. The Fixation, Metamorphpsis, and Further 

 Development of the L a r v se. 



General Remarks on the Fixation of the Larvae. — 

 There are two facts at least which tend to make the study of 

 the larvge of Spongilla during fixation and metamorphosis a 

 laborious task, and to render difficult a correct interpretation 

 of the phenomena observed. In the first place there must be 

 taken into account the difference in the structure of the larvae 

 at the time of fixation, a difference which is the result of 

 divergent variation culminating in the types C and D. In the 

 second place there are found other differences, due to the fact 

 that some of the larvae fix themselves at an earlier stage than 

 others ; such differences are exemplified by the types B and C. 

 In the former case we are confronted with a diversity of the 

 most fundamental nature, one in which the numerical propor- 

 tion of the several classes of cells which build up the inner 

 mass vary, and which no amount of delay with regard to 

 fixation can rectify. In the latter case the variation is not so 

 far-reaching in its effects, and the fixation need be delayed but 

 a very short time to obliterate them. 



The types of larvae which appear to be ripe for fixation are 

 C and D, but type B may occasionally settle down and undergo 



