34 THE ELEMENTARY NERVOUS SYSTEM 



the so-called dermal membrane (Fig. 7), is composed of 

 an external epidermis which is believed to be syncytical 

 and not cellular as heretofore assumed, of an intermedi- 

 ate mesenchyme containing spicules, and of an inner 

 epithelioid membrane forming the lining of the subder- 

 mal cavity. Each pore consists of two parts, the pore 

 or actual opening at the surface and the pore canal, a 



FIG. 7. Dermal membrane over a subdermal cavity in the sponge Stylotella seen from 

 the exterior as a somewhat transparent preparation. The outline of the subdermal cavity 

 is indicated by the line of limiting cells I, within which is the dermal membrane pierced by 

 six pores, three of which are partly closed by pore membranes. (After Wilson, 1910.) 



very short canal that leads through the thickness of the 

 dermal membrane to the subdermal cavity below. In 

 Stylotella the pores can be seen to contract and to close 

 by the activity of the epidermal syncytium (Fig. 8). 

 With the closure of the pore this layer forms over the 

 external end of the pore canal an extremely thin sheet, 

 the pore membrane, near the middle of which the pore has 

 disappeared. In this state the pore membrane is some- 

 what like the head of a drum, the pore canal representing 

 the body of the drum. In other sponges studied by Wil- 



