Sponge fauna of Norway. 435 



General Form. — If we imagine a round or oval tureen, 

 with a conical cover overlapping it at the edges, and the foot 

 produced into a number of descending rootlets, we shall have 

 a good idea of the general form of a symmetrically-grown 

 and adult example of Thenea WalUchii. The part corre- 

 sponding to the cover we shall call the upper half, that to the 

 dish the lower half of the sponge ; and the space between 

 them overlapped by the edge of the cover we shall call the 

 " equatorial recess." The upper half is usually conical, with 

 a circular oscule at the apex ; near the base it curves over 

 into a convex overlapping edge, which covers, as the edge of 

 a thatched roof does the eaves, the rounded annular inflection 

 which we term the equatorial recess. The lower half, which 

 is usually either more or less hemispherical or conical, is pro- 

 duced into a number of descending conical processes, from 

 each of which issues a root as a single fibre, which afterwards 

 frays out into a white woolly-looking tuft by the separation 

 of its component spicules. Variations, greater or less, from 

 the general form are very numerous : the equatorial recess, 

 which in the most symmetrical forms extends all round the 

 sponge, in others frequently fails to do so, being interrupted 

 at intervals, through which the upper and under surfaces pass 

 insensibly into each other ; sometimes it is confined to one 

 quarter of the circumference ot the sponge, or even less ; and 

 in one specimen, in every other respect precisely like its 

 fellows, it is entirely absent. The roots vary in number : in 

 the youngest specimens they are never more nor less than 

 one ; in the largest of Mr. Norman's specimens there are as 

 many as twenty ; on the other hand, in one remarkable 

 specimen of average adult size there are no roots at all, nor 

 any signs of their ever having been present. The roots are 

 liable to be given off from abnormal regions : thus, in a speci- 

 men from North America, dredged between Anticosti and 

 Gaspe, they arise from one side of the sponge at a place where 

 the equatorial recess would usually be present, but which has 

 been suppressed here and on the adjacent margin, with a 

 compensating over-development on the side opposite ; this 

 arrangement would lead to the sponge being so anchored or 

 rooted that the equatorial recess, which is a special pori- 

 ferous area, would be the uppermost part of the sponge, while 

 the oscule would lie halfway down tire side, looking out late- 

 rally. A similar modification occurs in another specimen 

 from the same locality, but v/ith a slight difference, which 

 leads to the oscule being situated on one side of the sponge, 

 and the limited equatorial recess on the opposite side, while 

 the roots descend from what appears to be the base, but 



