xxii THE VOYAGE OF H.M.S. CHALLENGER. 



means all, Monaxouid sponges unites the individual spicules into a coherent skeleton, and 

 ultimately (in the so-called Keratosa) constitutes by itself the entire skeletal system. In 

 chemical composition spongin has been found to resemble silk. Krukenberg has 

 recently investigated it, and assigns to it the chemical formula CsoHisNgOig.^ Its physical 

 characters and mode of occurrence in the sponge, however, concern us more in this 

 place than its chemical composition. It is usually pale yellow or amber-coloured, and is 

 arranged in concentric layers around the spicules which it unites together. Concerning 

 its mode of formation in any sponges but true so-called Keratosa we have no actual 

 observations. There can, however, be no doubt, from the analogy of the true Keratosa, 

 and from its arrangement in concentric layers, that it is a secretion of glandular cells 

 (spongoblasts). Our own histological investigations on the Challenger material throw 

 some light upon this point. We have found, in a number of different Monaxonid 

 sponges, sheaths of more or less fibrous connective tissue cells accompanying the 

 fibres of the skeleton. This condition has been carefidly observed by us in the following- 

 species: — Es]periopsis challengeri, Axinella (f) paradoxa, Raspailia tenuis, Acanthella 

 pidcherrima and Suherites perfechis. 



In Esperiopsis challengeri, where there is a fair amount of very pale coloured 

 spongin, these cells are fairly abundant ; they accompany the larger bands of spiculo- 

 fibre in dense tracts, in which the individual cells lie close together side by side. They 

 are fusiform, but apparently not usually A^ery much elongated, measuring about 0*024 

 by 0*0096 mm.; occasionally, however, they seem to be drawn out into a fine, 

 transparent thread at each end ; they are highly granular and stain well with 

 borax-carmine. 



In Raspailia tenuis the cells in question are uncommonly well developed ; and here 

 there is also a very large amount of spongin. They occur in a thick but not densely 

 packed sheath around the central axis, with which their longer axes are of course 

 parallel (vide PI. XLIX. figs. 1, 1«) ; they closely resemble those of Esperiopsis 

 challengeri just described, and their form will be best understood from the figures. 

 They average in size about 0"033 by 0'0096 mm. We have not succeeded in 

 distinguishing a nucleus ; the cells appear to be highly and uniformly granular throughout. 



In Acanthella p)idGherrima we have found similar granular cells scattered about 

 fairly plentifully amongst the spicules, but not forming a definite zone around a central 

 axis. The amount of spongin present is very small, but our thin, stained sections have 

 demonstrated its presence beyond a doubt. Hence we cannot entirely agree with 

 Schmidt, when he says ^ in his generic diagnosis " Parenchyma spisse impletum spiculis 

 simplicibus longioribus, substantia firmiori non inclusis." 



In Axinella [?) paradoxa, the fibrous tissue accompanying the bands of spicules is 

 very highly developed (PL XLIX. fig. 2a). It is composed of very much elongated, 



' Cf. Vosmaer, Bronn's Klass. n. Ordnuiig. d. Thieirreichs, Porifera, p. 434. ^ Spong. Adriat. Meeres, p. 64. 



