46 SPONGES 



trivances are very common in Tetractinellids, and in their most 

 elaborate form consist of a ring -like sphincter for closing the 

 aperture, and a layer of radially arranged elements for opening it. 

 In most cases only the sphincter is present. The cells resemble 

 those of the flat epithelium in all respects except in form and 

 position ; in fact, it must be confessed that the fact of their being 

 distinct and separate from the epithelium has often been assumed, 

 on the theory of a mesoderm, rather than demonstrated. 1 



Distinct glandular elements are not marked out in Calcarea, 

 except perhaps in the more columnar form of the epithelium where 

 the sponge is in contact with the substratum. In Demospongiae 

 separate gland cells are often present, having, as has been said, a 

 peculiar mushroom-like form. These cells are of special interest, 

 since from them, it would appear, are derived the spongoblasts of 

 the spongin fibres, which by their secretion form a very important 

 addition to the skeleton. 



The nature of the mushroom-like gland cells has frequently been 

 misunderstood, it having been supposed that both the external disc-like 

 portion and the more internal stalk contained each a nucleus of their 

 own. In this way two cells were made out of one an external flattened 

 cell supposed to belong to the dermal epithelium, and a more internal 

 glandular cell, decorated with processes of various kinds, considered 

 as mesodermal or " subepithelial " in nature (von Lendenfeld). The 

 external nucleus figured by this author is, however, non-existent, and the 

 whole cell belongs to the dermal epithelium. In many cases, indeed, 

 e.g. in Calcarea, the cells described as glandular are simply cells of the 

 flat epithelium in a contracted state. 



The spongoblasts are found as a sheath or " mantle " investing 

 the growing spongin fibres. Each spongoblast is of columnar form 

 (Fig. 50), resembling a mushroom -like cell of the epithelium, 

 without, however, the terminal disc. In Dictyoceratina the spongo- 

 blast layer surrounding the fibres is said to 'be continuous with the 

 epithelium at the surface of the body, where the tip of the fibre 

 raises the outer skin. The question of the origin of the spongo- 

 blasts is one which is, however, urgently in need of renewed in- 

 vestigations, current theories being based more upon assumptions 

 than upon observations, as in many other questions of sponge 

 histology. When the spongin fibres are fully formed, the glandular 

 spongoblast mantle disappears, its cells becoming, according to 

 Schulze (1879), stellate cells of the skeletogenous layer. 



Before leaving the epithelium there remains for consideration the 

 question of nervous elements in sponges. The existence of special 



1 Thus Merejkowsky (1878) describes in Halisarca a muscular sphincter of fusi- 

 form cells not covered by the "syucytium" ; in other words, composed of cells of 

 the flat epithelium. 



