PORIFERA 



23 



Ceratosa. — The embryos of Spongelia pallescens (Fig. 7) which are 

 ready to swarm possess a cylindrical form with one end convexly rounded 

 and with a shallow depression at the other. In the region of this shallow 

 invagination the flagellate cells are pigmented brownish red. The inside 

 of the embryo is filled with a gelatinous connective tissue. The embryo 

 of Euspongia officinalis before swarming presents a very similar struc- 









nt4 



Pig. 7.— Longitudinal section tbrough a larva of Spongelia pallesceus (a'^tcr 

 F. B. Schulze). a, pigmented epithelial cells of the posterior pole of the body; 

 ms, gelatinous connective tissue inside the larva. 



(The superficial covering of flagella has been omitted in the illustration.) 



thereby been acquired, inasmuch as the development of the Cornacu- 

 spongia fits in completely with that of Sycandra. The free-swimming 

 larval stage corresponds to the pseudo-gastrula stage of Sycandra. The 

 superficial flagellate epithelial cells of the larva subsequently become 

 the collared entoderm cells of the adult sponge ; whereas the inner cell- 

 mass, which in many cases is exposed at the posterior pole of the larva, 

 represents the common fundament of the ectoderm and mesoderm. The 

 ectoderm and mesoderm of sponges must be regarded as an undivided 

 whole. The larvae attach themselves by the anterior pole, and at the 

 same time occurs that inversion of the layers which must be compared 

 with the process of actual gastrulation in Sycandra.] 



