Reactions of Sponges y 



often form a definite layer around the whole finger diectlv under 

 the dermal membrane From these sub-dermal cavities pass off 

 the incurrent canals, which lead centrally into the flagellated cham- 

 bers. These chambers are usually spherical in form, measur- 

 ing about twenty to fortv micra in diameter and forming a more 

 or less compact layer surrounding the gastral cavity; they con- 

 nect with this cavity by short, irregularly branched, excurrent 

 canals. Where the gastral cavity is close to the external surface 

 of the sponge, there are apparently few or no ostia, sub-dermal 

 cavities, or flagellated chambers, but the dermal membrane and 

 the lining of the gastral cavity coalesce to form the translucent 

 wall already mentioned. In the living condition ot the sponge 

 the layer of flagellated chambers is orange in color while the other 

 parts of the animals are mostly dirty-yellow in tint. 



I have not studied with an}'^ fulness the histology of stylo tella 

 but in good osmic-acid preparations cut into sections ten micra 

 thick and stained in picrocarmine much of the cellular structure 

 of this animal can be made out. The outer surface of Stylotella 

 is covered with a dermal epithelium composed of polygonal cells 

 that are usually extremely thin, though at places they show a con- 

 dition approximating that of a cuboidal epithelium. The sub- 

 dermal cavities, incurrent and excurrent canals, and gastral cavity 

 are lined with a very flat epithelium, whose presence is difficult to 

 demonstrate unless it is well preserved and cut at a favorable angle. 

 The flagellated chambers are of course lined with a layer of rela- 

 tively large choanocytes. In some places the dermal membrane 

 seems to be made up of nothing but the dermal epithelium and 

 the lining epithelium of the sub-dermal cavity and is therefore 

 extremely thin. In most regions, however, it contains several 

 other kinds of cells. Of these one set is represented by elongated, 

 spindle-shaped cells, the so-called myocytes, and these are arranged 

 like irregular sphincters around the ostia. They also surround 

 abundantly the sub-dermal cavities, gastral cavity, osculum, etc. 

 Structurally they have the appearance of a primitive kind of 

 smooth muscle-fiber. In some places in my preparations they 

 seem to lie directly on the exposed surfaces of the canals and 

 cavities that thev bound as though they were merely elongated 



