196 SIGMATEIiLA.. 



of slightly curved tubes, which grow up from a common base. These tubes attain 

 a maximum width of about 40 millim. a little way over the base, aud taper from 

 thence gradually towards the upper end, which rarely exceeds 15 millim. in 

 thickness, and on the summit of which the circular osculum, 6-8 millim. wide, 

 is situated terminally. This is the most frequent form assumed by iS. a. 

 tubaria. It may, however, occur that these tubes arrange themselves in one 

 perpendicular plane and coalesce for the greater part of their length, thus 

 producing a stout flabelliform structure on the undulating or lobed free margin, 

 of which the oscula are situated in a row. For these forms the variety S. a. 

 jiahellum has been established. Tlie surface is pretty smooth, or slightly 

 granular, shagreen-like. The oscula are 6-8 millim. wide, and confined to the 

 summits of the cylindrical tubes or the margins of the flabellar forms, where 

 they always occupy the summits of the processes in case the margin is lobed 

 or dentate. 



The specimens referable to S. a. tubaria are dull orange-yellow w^hen alive, 

 those of S. a. fiabellum pink. In the dry state and in spirit they all appear 

 grey. There is no difference between the two varieties, except in their shape 

 and colour, so that the following anatomical description applies equally to both. 

 S'ujmatella anstraUs is comparatively tough and can be compressed without 

 breaking wheu alive, but it is fragile and more or less friable in the dry state. 



The skeleton consists of a network of fibres, composed of dense masses of 

 sand-grains, uniformly 0'03 millim. in size, which are held together by a very 

 small quantity of spongin. These sand-grains are so well packed that the 

 surface of the fibres appears smooth. From the base of the sponge longitu- 

 dinal fibres, on an average 0*1 millim. thick, extend upwards parallel to the 

 large oscular tubes, which terminate in the vents above described. None of 

 these fibres approach the outer surface or the wall of the oscular tube ; they 

 are confined to the intermediate part of the sponge, and are about 0*5 millim. 

 apart. They are connected by simple transverse fibres, 0-04-0-08 millim. thick. 

 From the main fibres branches of similar dimensions are given off towards 

 the outer surface, and also in the direction of the oscular tube. These 

 repeatedly anastomose and form a network with very irregular meshes, about 

 0-3 millim. wide. The outer surface is protected by a cortex, 0-1 millim. thick, 

 of densely packed sand-grains, which are similar to those in the fibres. A 

 single layer of small sand-grains underlies the wall of the oscular tube. There 

 do not appear to be any scattered sand-grains in the interior, although such 

 have been described by Marshall, who gives a somewhat different account of 

 the structure of the skeleton. The ends of the fibres of the supporting-skeleton 

 are joined to the cortex. Proper spicules are observed in the ground-substance ; 

 these are of two kinds : — A. Eods : rather scarce, and apparently confined to 

 the superficial layer, 0-12 millim. long and 0-002 millim. thick. Most of these 

 are rounded at both ends — strongyla, but styli and tylota are also occasionally 



