No. 3-] DEVELOPMENT OF MARINE SPONGES. 349 



the parenchyma or mes-entoderm of the attached sponge, sub- 

 sequently becoming connected into a continuous system. As 

 regards the development of the canal system, such varying 

 accounts are given by different authors that, were it not for 

 the help lent by comparative anatomy, it would be quite 

 impossible to form any idea of the fundamental morphology 

 of sponges. Fortunately for the student entering this puzzling 

 domain, comparative anatomy has in the hands of Haeckel, 

 Schulze, and Polejaeff provided a standpoint from which the 

 varying phenomena of development and structure may be 

 viewed with at least a partially understanding eye. It may 

 be that an increasing accumulation of facts will show that 

 Haeckel's conception of the relation of the simple calcareous 

 sponges to the complex horny and silicious forms is not well 

 founded, and that Schulze's view of the parts played by the 

 embryonic layers in producing the adult anatomy is not the true 

 one. But at present it is only with the aid of these theories 

 that one can form any clear conception of sponges in general, 

 and so for the present at least we are bound to accept them. 



Comparative anatomy points in no undecided manner to the 

 phylogenetic path along which sponges have developed, and so 

 permits us to construct a standard of ontogeny, with which we 

 may compare the actual development of each species as we 

 witness it to-day, and so be enabled to note the amount and 

 kind of divergence (coenogeny) exhibited. That coenogeny is 

 exhibited to a great degree in the embryology of sponges is 

 evident from the various types of development described, and 

 in the future much may be hoped from the study of a group 

 like this for the understanding of the laws of development. 

 For the present all we can do is to accept what seems the 

 most probable phylogeny, recording the instances of supposed 

 coenogeny as they are observed. Adopting this method, I 

 have to regard the development of Esperella and Tedania(/.f., 

 the later development or metamorphosis) as far removed from 

 the phylogenetic path. Before pointing out the pronounced 

 coenogeny exhibited in the development of these sponges, it 

 will be worth while to review briefly the evidence on which 

 rests the current view of sponge morphology. 



