SPONGES 29 



cells of the gastral epithelium, and towards the exterior by a fine 

 opening in a delicate, protoplasmic diaphragm situated on a level 

 with the dermal epithelium (Fig. 42, A, B, C, and D, d.a). 



Both scleroblasts and pore cells are derived directly from the dermal 

 epithelium which in the embryo at first constitutes the whole of the 

 dermal layer. Cells of the epithelium migrate inwards to become sclero- 

 blasts ; other epithelial cells, distinguished by their larger size and 

 numerous granules, become porocytes in two different ways in different 

 regions. In the oscular rim the epithelium lining the interior becomes 

 modified as it approaches the gastral layer, until its cells have the 

 characters of porocytes (Fig. 42, A, D, p.c.ep). As the collared epithelium 

 grows upwards by proliferation of its cells during the growth of the 

 sponge, the lowermost epithelial cells of the oscular rim become sur- 

 rounded by collar cells which pass between them and isolate them from 

 one another. Each cell of the lining epithelium of the oscular rim when 

 thus cut off from its fellows becomes a pore cell. In other regions of the 

 body wall the ranks of the porocytes may be recruited by the direct im- 

 migration of large granular cells of the dermal epithelium, and their 

 subsecpient perforation to form new pores. 



In addition to the collared cells of the gastral layer and the 

 various cell elements of the dermal layer, the body wall contains 

 numerous wandering cells or amoebocytes (Fig. 42, H, D, E, F, am.c), 

 which occur everywhere amongst the cells and tissues. Though 

 lodged principally in the dermal layer, they are not to be regarded 

 as belonging to it, but as constituting a distinct class of cells in 

 themselves. They are concerned probably with the functions of 

 nutrition and excretion, and from them arise the genital products. 



The above description of the Olynthus applies to it in the 

 normal expanded condition, when the sponge is feeding actively, 

 with pores and oscula widely open. The cells of the flattened 

 dermal epithelium, however, as well as the porocytes, are excessively 

 contractile, and by their contraction bring about important modi- 

 fications in the appearance of the sponge as a whole, as well as in 

 the disposition of its cells and tissues. Each porocyte can close up 

 its apertures and obliterate its lumen by its own contraction, and 

 in this condition the porocyte has the appearance simply of a com- 

 pact, granular, amoeboid cell. The contraction of the dermal 

 epithelium brings about the closure of the osculum and the con- 

 traction of the sponge as a whole. The closure of the osculum is 

 effected more especially by the large granular epithelial cells, 

 destined to become porocytes, which line the oscular rim, and from 

 these cells a special contractile apparatus, such as a ring -like 

 sphincter or a contractile sieve-membrane, is often formed in this 

 region. The flat epithelium covering the exterior, on the other 

 hand, is responsible for the general contraction of the whole body, and 

 by its action brings about a reduction in the internal gastral cavity, 



