SPONGES 141 



in amount, and possibly absent altogether in some cases. In the 

 hollow or heterogeneous fibres, on the other hand, the medullary 

 substance is largely developed, making up often the bulk of the 

 fibril, but relatively less abundant in the older fibres than in the 

 younger. 



In form the spongin fibres are usually cylindrical, but may be 

 slightly compressed and even flattened or leaf-like in places 

 (Dendrilla). The growing portions of the spongin fibre are 

 enveloped in a sheath or "mantle " of spongoblast cells, of columnar 

 epithelial form, which appear to deposit concentric layers of spongin, 

 as a cuticular secretion, upon the surface of the fibre. Many 

 details of the growth remain, however, obscure and in need of 

 further investigation, especially as regards the origin of the 

 medullary substance. 1 



When the fibres have attained their definitive growth, the 

 spongoblasts seem to disappear, perhaps becoming converted into 

 connective tissue cells. 



As regards the arrangement of spongin fibres to form the 

 skeleton as a whole, two types can be distinguished, the reticulate 

 and the dendritic. In the reticulate type the skeleton is made up 

 of a continuous network of anastomosing fibres, in which principal 

 and connecting fibres can be distinguished. The former (Fig. 92, D, 

 m.f) run vertically upwards to the surface and raise it up into 

 little tent-like projections or conuli. The connecting fibres take 

 a more horizontal course. In the dendritic type, characteristic of the 

 family Aplysillidae, the skeleton consists of heterogeneous fibres which 

 grow upwards like a tree from a basal plate of spongin, branching 

 freely, but remaining distinct from one another. The terminal 

 branches raise the skin into cenuli. In the genus Darwinella a 

 skeleton of this kind is found combined with separate spicules of 

 spongin having the same structure as the fibres of the skeleton. 

 The spicules in question are of variable form, but in many cases 

 distinctly of a six-rayed or triaxon type ; the rays vary, however, 

 from two or three to as many as eight, and the angles at which 

 they meet are irregular and inconstant. Nothing is known 

 regarding their origin and formation. 



The property possessed by many sponges of taking up foreign 

 bodies into their fibres has already been noticed (p. 42). In the 



1 According to Lendenfeld, whose results require confirmation, the medullary 

 substance in Dendrilla owes its origin to cells derived from the spongoblast layer, 

 which become included in the fibre at its growing point. The function of these cells 

 is supposed to be the production of medullary substance by destruction and modi- 

 fication of the layers of cortical spougin secreted by the enveloping spongoblasts, and 

 they are hence termed by Lendeufeld " spongoclasts," on the analogy of the marrow 

 cells or osteoclasts of Vertebrata. Cells are also stated to occur in the horny fibres 

 of the genus lantkella, but in this case they are found between the spongin lamellae 

 of the cortical layer, and not at all in the medullary substance. In no other cases 

 have cells been observed in the interior of the fibres. 



