REPORT ON THE MONAXONIDA. xxiii 



rather slender, fusiform cells. These cells are highly granular, and often multipolar, 

 differing really but very slightly from the ordinary mesodermal cells which occur in the 

 ground tissue of this sponge, save for their great elongation in one particular direction 

 {cf. PI. XLIX. fig. 2). They commonly measure about 0'048 by 0-007 mm., and in some 

 cases there appears to be a nucleus in the centre of the cell. The spongin uniting the 

 spicules together is distinct but not very abundant. 



In Suberites perfectus there is, as is usually the case in the Clavulina, no trace of 

 spongin. But the skeleton fil:)res are accompanied and apparently held together l^y 

 dense bands of very strongly fibrous connective tissue (PI. L. figs. 2, f.t, 2a, 2b). 

 Scattered fairly abundantly through this tissue are a number of fusiform, elongated, 

 more or less granular bodies, more numerous in some parts than in others (PI. L. figs. 

 2«, 2b, g). The bands of fibrous connective tissue are directly continuous with the 

 fibrous cortex. 



The data before us are perhaps too scanty to justify any wide generalisation, but 

 it is a significant fact that in Suberites perfectus, where there is no spongin to hold the 

 spicules together, the fibrous element of the surrounding connective tissue is very strongly 

 developed, and the granular element comparatively feebly developed, while in Raspailict 

 tenuis, in which there is a very great deal of spongin, and hence no need for fibrous 

 tissue to unite the spicules, exactly the reverse is the case. The other species 

 mentioned, viz., Esperiopsis challengeri, Axinella {?) paradoxa and Acanthella 

 pulcherrima, occupy intermediate positions between these two extremes, both with 

 regard to the amount of spongin present and with regard to the degree of develop- 

 ment of the granular and fibrous elements of the connective tissue respectively. 



In explanation of these facts we would venture to suggest that in Raspailia tenuis 

 and other species where spongin is present, surrounded by highly granular, specially 

 modified, mesodermal elements, the latter secrete the former, and were themselves 

 originally ordinary mesodex'mal cells, which have become slightly modified for the 

 fulfilment of this special function. In other sponges {e.g., Dendrilla), where there is 

 a still greater development of spongin, these cells have become still more highly 

 specialised and constitute the " spongoblasts " of the so-called Keratosa. These 

 suggestions are in perfect harmony with the views now generally held as to the late 

 appearance of spongin in the Porifera. 



To sum up briefly, we may state that spongin is a secretion of mesodermal cells, 

 which, originally indistinguishable from other mesodermal cells, have become under 

 suitable conditions sjjecially modified in form in order to fulfil their special function ; 

 and it is probable that, in at any rate some cases, these cells were derived from 

 bands of fibrous connective tissue which accompanied the bands of spicules for the 

 purpose of holding them together. 



Coincidently with the development of the spongin the filirous tissue in some cases 



