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(z.e., 
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. 
