SPONGES 



47 



epithelial or sub-epithelial nerve cells has been affirmed by Stewart 

 (1885) for Grantia compreissa, and by Lendenfeld (see especially [8]) for 

 various sponges. Sollas also cautiously suggested a similar interpretation 

 for certain elements observed in or near the sphincters of Tetractinellids, 

 and proposed for them the term aesthacytes. No proof was at any time 

 brought forward, however, as to the nervous nature of the structures in 

 question, and at the present day the existence of any special nervous 

 apparatus in sponges has become universally discredited, partly because 

 subsequent investigations have been unable to confirm the alleged dis- 

 coveries, and partly because some of the structures supposed to be 

 sensory receive a simpler explanation in another way. For instance, the 

 so-called "palpocils" and "synocils," described in Calcarea by Stewart 

 and Lendenfeld, can easily be found in preparations of these sponges, 

 especially if mounted in glycerine, as already noticed by Lendenfeld 



FIG. 50. 



Growing spongin fibre, with spongoblasts attached (after Schulze). x550. spj, spongin 

 fibre ; sp.bl, spongoblasts ; Coll, collencytes. 



(1891). They are nothing more than portions of .the dermal epi- 

 thelium raised up into a tent -like elevation by the projecting ray 

 of a calcareous spicnle, which has become dissolved in the preparation. 

 In the interior of the papilla thus formed are seen the scleroblast or 

 formative cells of the spicule, spread over the spicule sheath and running 

 up to the tip of the ray ; and it is these elements, and perhaps some 

 others also, such as wandering cells, which have been erroneously 

 identified as sense cells. 



In the Olynthus there can be no doubt that the flat epithelium 

 performs sensory functions of an elementary kind, but it exhibits as little 

 special differentiation for this function as it does for that of contractility. 

 In Calcarea generally the same state of things is found ; reaction to 

 external conditions is manifested both by the porocytes and by the flat 

 epithelium, but the primitive condition of the dermal layer in this group 

 makes it almost certain that nerve cells do not occur here. Of Hex- 

 actinellids nothing can be stated definitely either way. In Demospongiae 

 it is not possible to deny positively a priori the existence of nerve cells, 



