A GUIDE TO THE SPONGES 225 



(a) The A scon Type (p. 2 2 1 , Fig i .) . This type is characterized 

 by sponges having walls with a thin layer of mesodermal tissue 

 {mes.), and therefore, with pores {p.) opening directly from the 

 outside into the paragastric cavity {air.). The endoderm 

 {end.) is always continuously lined with choanocytes or "collared 

 cells " (cho.). Ascetta primordialis, therefore, is the representative 

 of this group. Another example is Leucosolenia, of which a speci- 

 men may be seen in this alcove. A complication of this type is 

 shown by Homoderma, which differs from Ascetta in having its 

 surface broken up by a multitude of radially arranged thimble- 

 like prolongations or diverticula, each with a central cavity of 

 its own, opening into the main paragastric cavity of the sponge 

 and lined with a continuation of the endoderm with its collared 

 cells. In this case the pores are found only in the walls of the 

 diverticula. 



(b) TheSycon Type {^p. 221, Figs. 2-5; p. 224, Fig. 9). In this 

 type, as in the example just described {Homoderma), the walls of 

 the paragastric cavity are prolonged into radially arranged 

 branches called radial tubes {cc.) but the choanocytes, instead of 

 lining both the paragastric cavity and the radial tubes, are found 

 only in the latter, while the former is invested with a layer of 

 epidermal "pavement cells" {ect.) like the outside of the sponge. 

 The mouth of the radial tube by which it opens into the central 

 cavity is called the apopyle {apy.). In the simpler sponges of 

 the Sycon type, such as Sycon ciliatum, the pores open directly 

 into the radial tubes (Fig. 2) and the outer surface of the sponge 

 is covered with papillae corresponding to the cavities within. 

 In these forms, the mesoderm {mes.) continues to be thin. In 

 other forms, however, the mesoderm becomes greatly thickened 

 and completely fills the spaces between the radial tubes (Fig. 3) 

 so that the outer surface appears comparatively smooth and free 

 from papillae. Under these circumstances the pores cannot 

 open directly from the outside into the radial tubes, so they 

 lengthen into inhalent canals traversing the mesoderm. In still 

 other forms (Fig. 4) the canals have enlarged to wide cavities or 

 inhalent lacuna {inh.) opening to the outside by the pores and into 

 the radial tubes by openings called prosopyles {prp.). Another 

 complication occurs in the Leucons where the walls of the para- 



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