SPONGES 27 



The body form of the Ohjnthus is typically that of a hollow 

 vase, as described above, though it may vary a good deal in its con- 

 figuration. Fig. 2, (I, shoAv^s the Ol/jnfhus of Clathrmi cnriacea ; Fig. 1 

 represents somewhat diagi'ammatically, and more highly magnified, 

 that of an allied species, (7. primordiaUs ; and Fig. 60, /i, that of Sycon 

 raphanns. As a type for description may be taken that of a simple 

 Ascon (CUithrina). 



The wall of the Ohjnthm (Fig. 1) is perforated by numerous pores, 

 and at the summit is situated the large exhalant aperture or osculum, 

 often defended by a contractile sphincter or sieve-membrane. The 

 body wall is composed of two layers of tissue, Avhich may be termed 

 the dermal and gastral layers respectively. The dermal layer is 

 the more externally situated and makes up the greater part of the 

 sponge. The gastral layer lines the interior, but does not reach 

 quite to the extreme margin of the osculum, the opening of which 

 is surrounded by a rim or collar of variable length, made up of the 

 dermal layer alone (Fig. 42, A and I), p.c.eji). Both layers are inter- 

 rupted by the pores, which perforate the wall everywhere except at 

 the base of attachment and in the oscular rim. 



The gastral layer is very simple in its composition, being made 

 up of a single stratum of columnar epithelium, the cells of which 

 are all of one peculiar type (Fig. i2, A and I), ch.c). Each cell 

 bears at its upper free extremity a single vibratile fiagellum (/?), 

 which springs from the centre of an area enclosed by a delicate cup 

 or collar of protoplasm (c). On account of the latter peculiarity 

 these cells have been termed collar cells or choanocytcs, and are very 

 characteristic of sponges. In all sponges that have been studied 

 the gastral layer is composed of these cells and of these alone ; on 

 the other hand, similar cells are not known to occur in any 

 ^Metazoa, but each collar cell is strikingly similar to a protozoon 

 individual of the class Choanoflagellata. 



The dermal layer consists mainly of a gelatinous ground sub- 

 stance, which is covered on all its exposed surfaces — that is to say, 

 on the exterior of the body wall and in the oscular rim — by a 

 fiattened epithelium {d.ep), and contains the skeletal elements and 

 their secreting cells and the pore cells. The flattened epithelium 

 is the contractile layer of the sponge, and where the body wall is 

 in contact with the substratum at the points of attachment, the 

 epithelium is of a glandular nature. The skeleton consists, in Cal- 

 carea, of spicules of calcite (qj) secreted within cells termed sclero- 

 blasts {xp.c). Each pore {p) is a perforation through a single cell, 

 the pore cell or porocyte {p.c), which stretches from the external 

 flat epithelium to the internal layer of collar cells, and jJaces the 

 gastral cavity in communication with the exterior by means of an 

 intracellular duct or canal. The pore canal opens towards the 

 interior by a wide aperture (Fig. 42, A and D, g.a) between the 



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