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



bearing surface. Indeed, so much does the appearance of the upper surface differ according 

 to whether the sponge be contracted or expanded, that Sir Wyville Thomson recognised 

 a distinct species for each of these two conditions.^ The figures (PI. L. figs. 3, 3a) repre- 

 sent the sponge in a partially contracted condition. Judging then from these facts, 

 we are probably correct in assigning to the fibrous tissue of the ectosome a muscular 

 function. In Tentorium the thick, fibrous ectosome is present only on the upper, pore- 

 bearing surface (PL L. fig. 3, ^j.«.). Elsewhere the columnar body of the sponge is 

 encased in a cylinder of densely packed, vertically disposed, large tylostyli. Inside of 

 this sheath of spicules comes a thin layer of gelatinous mesoderm, containing stellate 

 ceUs, and inside this again a very thin zone of fibrous tissue in which the fibres are 

 arranged in a circular manner, running round and round the sponge ; very possibly these 

 circular fibres also are muscular in function ; but, judging from then- slight degree of 

 development, we cannot suppose that they are very powerful. 



The fibrous tissue of the ectosome appears to be always very much the same in histo- 

 logical character, composed of very slender, very much elongated cells, closely packed 

 together, with elongated nuclei scattered here and there. The same kind of tissue is not 

 infrequently found surrounding the larger exhalent canals {e.g., Stylocordyla stijntata, 

 var. globosa, nobis ; Latrunculia apicalis, nobis) and in these cases also it ought possibly 

 to be regarded as part of the ectosome. 



As in the case of a gelatinous ectosome, there are usually several elements entering 

 into the composition of the fibrous cortex. Thus, in Stylocordyla stipitata, var. glohosa, 

 there are present an enormous number of rounded or oval, nucleated cells (PL L. fig. la, 

 g.c.) lying in and amongst the fibrous tissue; each cell measures about 007 mm. in 

 diameter, is somewhat granular and stains fairly deeply with borax-carmine. They are 

 most abundant in the outer portion of the cortex, being closely packed together and 

 making up by far the chief portion of the ectosome in this place. What their function 

 may be is very doubtful, but fi-om their form and their position in the sponge we are 

 inclined to place them in the category of "glandular (?)." We have observed similar, 

 but more highly granular and deeply staining cells in the ectosome of Suherites 

 ramulosus, var. cylindrifera, nobis. 



In Latrunculia apicalis the ectosome has a somewhat diff"erent composition. In as 

 much as it is tough and firm and sharply marked off from the unc^erlying choanosome 

 {vide PL LI. fig. 1), and contains, moreover, a considerable amount of fibrous tissue, it 

 may be considered as forming a true cortex. Here, as is also the case in the genus 

 Suherites, its toughness and firm consistency are largely due to the presence in it of 

 numerous supporting spicules, the arrangement of which is described elsewhere, and 

 wiU be seen by reference to the figures. There is a thin external layer of somewhat 

 fibrous tissue (PL LI. fig. Ih, f.t.), in which are embedded the bases of the closely- 



1 The Depths of the Sea, figs. 23, 24. 



