No. 3-] DEVELOPMENT OF MARINE SPONGES. 359 



indicate tliat 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 

 (ibid., 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 engnlfcd by mesoderm cells, 

 and then become the lining cells of the flagellated chambers ! 



