5H 



WILSON. 



[Vol. V. 



conspicuous because of the pigment (orange) which covers the 

 rest of the embryo. In this condition, the embryo breaks 

 through its follicular wall, and passes out of the parent through 

 one of the oscula (Fig. 3). 



The first step in the metamorphosis of the larva consists in 

 the flattening of its ectoderm. The flattening begins before 

 the sponge attaches itself, and gradually travels from the non- 

 ciliated or spicular pole backwards. While there is still a con- 

 siderable remnant of the columnar ectoderm, the larva attaches 



Fig. 3. Longitudinal section through the swimming larva of Esperelia. The 

 nuclei of the columnar ectoderm cells form a conspicuous zone. In the outer ends 

 of these cells is orange pigment. The inner mass of cells (parenchyme), in the 

 region of non-ciliated pole, differs from the parenchyme in the rest of the body. 



itself by the spicular pole, but obliquely, so that it lies some- 

 what on its side. The rest of the ectoderm then becomes 

 flattened, and the larva is transformed into a thin, flat mass, 

 circular in outline. The bundle of spicules, formerly at the 

 non-ciliated pole, become distributed all over the body of the 

 little sponge. The attached larva, at first circular in outline, 

 speedily grows irregular in shape, and becomes surrounded by 

 a thin ectodermal membrane. The subdermal cavities and 

 canals appear as lacunae in the substance of the larva, the sur- 



