SPONGES 43 



one such instance, Spirastretla decumbens, R. and D., upgrowths from 

 the basal plate, are said to give rise to a lamellar supporting skeleton 

 (Keller, 1891). Where an internal fibrous spongin skeleton exists, 

 it may be supposed to originate in the first instance either from the 

 upper surface of the sponge body by an ingrowth of spongoblasts 

 from the epithelium, or as an upgrowth from a basal spongin plate. 

 An origin of the first kind would explain the very frequent inclusion 

 in the fibres of foreign bodies of all kinds, which would be absent 

 in fibres derived in the second way ; two possibilities which appear 

 to be realised in the two orders of horny sponges (see below). 



In the case of the elastic fibrillae, on the other hand, the secre- 

 tion is intracellular, and comparable to the formation of spicules 

 (see below, p. 50). "We thus have an interesting case of a skeletal 

 substance being laid down either as a spicular (intracellular) con- 

 cretion or as a cuticular (extracellular) cement. These two forms 

 of sjiongin secretion run parallel to the two forms of mineral 

 (siliceous) deposits already mentioned. 



It must be acknowledged, however, that the details of the secretion 

 of the spongin fibres still remain to be clearly investigated. Their 

 cuticular nature is inferred from the relations of the spongoblasts to the 

 fibres (see Fig. 50), and from the fact above mentioned of the similarity 

 and even continuity between fibres and cuticle. 



The apparent parallelism between the secretion of spongin and of 

 silica suggests strongly the possibility of an interchange taking i^lace 

 between these two forms of skeletal material, whereby one might become 

 substituted for the other in a given instance. Similarly in Acanthometridae 

 the siliceous skeleton of other Eadiolaria is replaced by an Acanthin 

 skeleton (see Protozoa). Such a substitution is further indicated by 

 the spongin spicules of Dancinella (see below, p. 141), upon which investi- 

 gations are urgently needed to throw light upon this point. 



An aberrant type of sjiongin secretion is said to occur in Stelletta 

 siemensi (Keller, 1891) in the form of spherical or oval bodies, each in a 

 follicle-like cavity surrounded by a layer of epithelial cells ; but some 

 scepticism is perhaps permissible as to the true chemical nature of these 

 bodies. 



(c) Ilidolocjij. — It has already been seen that the Olynthus of a 

 simple calcareous sponge is composed of five classes of cells ; four 

 of these, namely, flat epithelial cells, skeletogenous cells, collared 

 cells, and archaeocytes, are found in all sponges, each giving rise to 

 several sub-classes. Porocytes have not, however, been recognised 

 as yet in sponges other than Calcarea as clearly as could be 

 desired. 



(1) Dermal Epithelium. — In all sponges an external layer of 

 flattened epithelium is present, though it may apparently degener- 

 ate in places into a cuticular covering. "With a few exceptions the 

 nature of this epithelium is remarkably uniform, consisting of a 



6 



