72 CCELENTERATA 



The simplest sponges (Fig. 132, A) are usually cylindrical struc- 

 tures, either colonial or not, in the walls of which are numerous pores 

 through which water streams into the cloacal cavity; the osculum is at 

 the free end of the body. The body wall is composed of three layers, 

 the outer ectoderm or dermal epithelium (Fig. 133,1), the middle skeletog- 

 enous layer or mesoglea containing the skeletal elements (2), and the 

 entoderm (3) consisting of peculiar cells called collar cells or choano- 

 cytes which line the interior cavity. Each collar c'ell is provided with a 

 single flagellum, the base of which is surrounded by a high ridge or 

 collar. 



This simple structure is called the ascon type of sponge. Other 

 sponges have what is called the sycon type of structure (Fig. 132, B). 

 In this the middle layer is much thicker than in the aseon type and from 

 the central cavity numerous cylindrical diverticula called the radial 

 canals (Fig. 132, B, 4) extend into the walls, and communicate also with the 

 outside through pore canals (3). In these sponges the 

 collar cells are confined to the radial canals, the central 

 cavity being lined with a flattened epithelium. Still an- 

 other type of sponge (Fig. 132, C) is called the leucon 

 or rhagon type, in which the skeletogenous layer is still 

 Fig. 133 thicker than in the sycon type and the collar cells are 



sponge 7Lenden- Confined to widened portions of the radial canals called 

 ^derm (^collar"" ^^'^ flagellate chambers (5). 



Dortln^ ?a ^"r^- ^^^^ great majority of sponges belong to the leucon 



3, entoderm. type; in these the middle layer constitutes by far the 



greater part of the body of the animals. 

 The ectoderm forming the dermal epithelium in all sponges is a 

 single layer of flattened cells which in a few cases (Oscarella) is ciliated. 

 In many sponges the ectoderm is more or less glandular and in all it is 

 contractile, the contractile elements in it being elongated cells called 

 myocytes which form sphincters around the pores and oseula and often 

 also surround the cloacal and other cavities; the ectoderm is also some- 

 times sensitive. 



The mesoglea varies much in thickness in different sponges, being 

 generally thin in the smaller and more primitive sponges and thick in 

 the higher and larger ones. It arises as a secretion of the ectoderai 

 and contains various cellular elements, and usually also calcareous 

 or silieious spicules, or horn-like fibres composed of a substance called 

 spongin. 



The spicules are of a great variety of forms and fall into two gen- 

 eral groups which are called megascleres and microscleres (Fig. 134). 

 The former are usually elongate or radiate in foim and are often bound 



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