No. 3.] DEVELOPMENT OF MARINE SPONGES. 359 
indicate that the two layers are of much the same nature. 
This essential similarity between the two layers has always 
been maintained by Metschnikoff, not only on the ground of 
development, but for physiological, reasons as well. Thus in 
young Spongillas when the water became bad, he has witnessed 
the entire disappearance of the flagellated chambers, the sponge 
then consisting of ectoderm and mesoderm alone. With a 
fresh supply of water the chambers reappeared (12, p. 375). 
Again, after feeding carmine in an excessive amount to Hali- 
sarca pontica, he found that the canals and chambers entirely 
disappeared, the whole body of the sponge inside the ectoderm 
consisting merely of a mass of amoeboid cells full of carmine 
(¢b7d., p. 372). The development of the afferent system in 
Chalinula was not worked out with certainty. 
The embryology of the preceding sponges, in which a 
rhagon type is developed, agrees pretty well with our general 
notions of sponge phylogeny. But there are other sponges, 
the development of which has been so excessively modified as 
no longer to be of any use as finger-posts to phylogeny, but 
which afford an excellent field for the study of what may be 
called the methods of coenogeny. In Halisarca Dujardinii 
(Metschnikoff 12), for instance, there is a solid larva in which 
the canals appear as so many separate lacunae surrounded by 
parenchyma (mes-entoderm) cells. The canals only subse- 
quently acquire a connection with each other. 
In Esperia (Maas 16), the subdermal spaces, canals, and 
chambers arise separately as lacunae in the parenchyma. The 
chambers are formed from aggregations of small cells (which 
Maas believes, on what seems to me insufficient evidence, to 
be ectoderm cells of the larva that have migrated into the 
interior). The efferent canals, Maas thinks, are formed from 
similar cells. 
In Esperia, according to Yves Delage (36), the chambers 
arise by division of special mesoderm cells. The epithelium 
of the canals comes from the larval ectoderm, which migrates 
into the interior. In Spongilla, according to the same author, 
the ectoderm cells of the larva are exgulfed by mesoderm cells, 
and then become the lining cells of the flagellated chambers! 
