SPONGES 51 



and originate like them from the dermal epithelium, from which 

 also arises the spongoblast layer, which by its secretion cements 

 the spicules together. It is possible that the porocytes in siliceous 

 sponges have only a skeletogenous function, and have not acquired 

 any relation to the pores, but this question must at present be 

 considered an open one. 



(3) The Skeletogenous Stratum is developed to a very variable 

 extent in different sponges. Scarcely recognisable in some, in 

 others it attains great proportions, making up all but a relatively 

 insignificant portion of the total bulk of the sponge body. It consists 

 of a gelatinous ground substance or mesogloea (" maltha," Haeckel), 

 which contains cells of various kinds. The mesogloea is the first 

 portion to appear as a structureless layer between the dermal and 

 gastral epithelia, and is probably a secretion of the former. Cells 

 from the dermal epithelium next migrate into the mesogloea, form- 

 ing a parenchyma which is concerned primarily with the task of 

 furnishing skeletal structures for the support of the sponge body. 



The separation, however, between the contractile (epithelial) and 

 skeletogenous (parenchymatous) strata of the dermal layer does not 

 amount to a very hard-and-fast distinction. As regards the function of 

 secreting skeletal structures, we find not only that so important a con- 

 stituent of the skeleton as spongin owes its origin apparently to cells of 

 the epithelium which have secondarily passed inwards, but that even 

 spicules may be secreted by cells of the epithelium which remain 

 in their primitive position, as in Leucosolenia, Spongilla, and 

 probably in many other cases. Further, in Ascons, and very 

 probably in all Calcarea, the skeletogenous layer does not grow by 

 multiplication of its cells amongst themselves, but their number is 

 recruited throughout life by immigration of cells from the dermal 

 epithelium ; how far the same is true of other sponges has not been 

 investigated. Hence the distinction between the epithelial and skeleto- 

 genous tissues is rather one dependent upon a gradual specialisation 

 of function, differing in degree from one species to another, than upon 

 morphological characters of fundamental importance, and there is no 

 reason from the histological point of view for regarding the skeletogenous 

 tissue as constituting a special layer or " mesoderm " possessing the same 

 importance or independence as the dermal or gastral layers. 



The cellular elements of the parenchyma may be classified at 

 the outset into scleroblasts and connective tissue cells, the difference 

 between the two being primarily one of function, according, that is 

 to say, as a cell does, or does not, secrete a spicule. Of the two 

 classes of cells thus marked out only one may be present in a given 

 case. Thus in Ascons, and perhaps in Calcarea generally, con- 

 nective tissue cells are absent, and though they have frequently 

 been described, the cells which have been so interpreted are in 

 reality merely scleroblasts or formative cells which, in the process of 



