44 SPONGES 



single layer of flattened, plate-like cells (pinacocytes, Sollas), with a 

 large spherical or slightly compressed nucleus lodged in the thicker 

 central portion of the cell. Mutual contact between the cells pro- 

 duces a network, with polygonal meshes, of cell outlines, often 

 visible in the living condition, and usually demonstrable without 

 difficulty by means of the silver nitrate reaction. In a few ex- 

 ceptional cases the flat epithelium is ciliated, as in Oscarella, 

 Aplysilla, and perhaps in some other cases. 1 It is often covered 

 externally by a cuticle secreted by the cells. 



The form of the epithelial cells may become greatly modified, as has 

 been described, as the result of contraction, which may cause them to 

 assume a shape like that of a mushroom with a bulbous stalk the so- 

 called flask-shaped or onion-shaped epithelium. In most cases this form 

 is only temporary ; in a few instances, however, it would appear to be 

 the normal form of at least a part of the epithelium, especially where it 

 is of a glandular nature. In Halisarca the epithelium of the outer 

 surface but not that lining the canals is curiously modified in connec- 

 tion with the abundant secretion of mucus with which this form covers 

 itself. 2 



In the most primitive sponges, as has been seen in the Olyn- 

 thvs, the dermal epithelium performs a variety of functions while 

 remaining a uniform layer of cells. Apart from the fact that in 

 the lowest forms the skeletogenous layer is recruited from it, and 

 that its cells may even secrete spicules while retaining their epi- 

 thelial position, the dermal epithelium commonly combines con- 

 tractile (neuromuscular) and glandular functions. Thus in the 

 Calcarea sphincters or specially contractile organs are formed simply 

 of ordinary flattened epithelium. In the Hexactinellida we have 

 no evidence of any contractility. In the Demospongiae the 

 primitive condition may be retained or may be superseded in the 

 higher forms by a differentiation of the cell elements corresponding 

 to a physiological division of labour. A separation is effected 

 between more internally placed contractile elements and a more 

 external glandular and protective epithelium proper, and since in 

 the latter the glandular elements may become further differentiated, 

 we have two new groups of cell elements arising from the primitive 

 epithelial cell. 



The contractile cells or myocytes, Sollas ("contractile fibre 

 cells," Schulze), are fusiform cells, lying below the epithelium, and 

 often forming contractile mechanisms in connection with the larger 

 exhalant or inhalant openings of the canal system. Such con- 



1 Lendeufeld at one time figured in all cases the flat epithelium as flagellated, but 

 these flagella, with few exceptions, are to be regarded as " phantasms of the living." 



3 The figures of the epithelium given by Schulze (1877) for If. Dujanlini, and 

 by Merejkowsky (1878) for "//. Schnltzi," differ considerably. 



