The elementary nervous system elementarynervou00park Year: [c1919] 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
The elementary nervous system elementarynervou00park Year: [c1919] 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-
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