STRUCTURE OF THE LARVA OP SPONGILLA LAOUSTRIS. 365 



was made up of the lining of the larval cavity and its diver- 

 ticula extending into the " mesoderm," by which term he 

 included the remaining cells of the larva. He described the 

 larval cavity as being produced by liquefaction and breaking 

 down of some of the central cells. 



When the larva became fixed the "ectoderm" was said to 

 flatten out and to become the flat epithelium. The '^meso- 

 derm " was supposed to produce the connective tissue and the 

 wandering cells, while the ends of the diverticula of the "en- 

 toderm " gave rise to the flagellated chambers, the diverticula 

 themselves producing the exhalant canals, and the larval cavity 

 becoming the gastral cavity. 



The next observer who studied the development and meta- 

 morphosis of Spongilla was Gotte (5), who wrote in the year 

 1886. Gotte divided the cells of the larva into two classes, 

 which he called '^ ectoderm" and " entoderm." The '' ecto- 

 derm " consisted of the flagellated cells of the surface layer, 

 while the term "entoderm" included all the cells enclosed 

 within this layer. He derived the larval cavity by mere sepa- 

 ration of the cells at the centre, and held that the flagellated 

 cells were thrown away when the larva became fixed, and con- 

 sequently that the whole sponge was built up from the " ento- 

 derm." He was of opinion that the granules contained in the 

 large cells in the interior of the larva were yolk bodies, which 

 became filled with chromatin, and developed into nuclei, the 

 future nuclei of the cells of the flagellated chambers. Therefore, 

 according to Gotte, the flagellated chambers arose from the 

 large cells of the interior, — that is, the cells with vesicular 

 nuclei ; the remaining cells of the " entoderm " giving rise to 

 the rest of the sponge. 



The third observer to study the development of Spongilla 

 was Maas (7), who wrote in the year 1890. 



Maas describes the larva as consisting of " ectoderm," " me- 

 soderm," and " entoderm," terms which had the same meaning 

 as was given them by Gauin. The results of his investigations 

 agree to such an extent with those of Ganin as to need no 

 further statement of them than to say that he considered 



