32 JOURNAL OF THE 



development and structure may be viewed with at least a 

 partially understanding eye. It may be that an increas- 

 ing- accumulation of facts wnll show that Haeckel's con- 

 ception 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 

 the sponges in general, and so provisionally at least we 

 are bound to accept them. 



Comparative anatomy points in no undecided manner to 

 the phylogenetic path along which sponges have develop- 

 oped, 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) ex- 

 hibited. 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 pres- 

 ent all we can do is to accept wdiat seems the most probable 

 phylogeny, recording the instances of supposed coenogeny 

 as they are observed. Adopting this method, I have to 

 regard the development (?'. c.^ the later development or 

 metamorphosis) of Esperella and Tcdania as far removed 

 from the phylogenetic path. Before pointing out the fea- 

 tures in which the development of these sponges is so 

 strongly coenogenetic, it will be worth wdiile to review 

 briefly the evidence on which rests the current view of 

 sponge morphology. 



Evidence from Comparative Anatomy as to Sponge Phy- 

 logeny. The strongest evidence offered by comparative 

 anatomy lies in the series of forms, passing by gradations 



