TWO NEW SPONGILLJl FROM LAKE TANGANYIKA. 473 



the skeleton, though supporting the dermal membrane, do not 

 in the natural condition penetrate it. 



An osculum is situated at the tip of each of the lobes or 

 mound-like elevations of the surface of the sponge. This 

 opening measures about an eighth of an inch in diameter, and 

 underlying it there is a fairly large gastral cavity. The 

 dermal pores are small, as usual, and are situated on the flanks 

 of the lobes as well as in the intermediate depressions. 



(2) The Skeleton. — In treating of the skeleton or the 

 supporting part of the sponge, first, the spicules will be de- 

 scribed ; secondly, the arrangement of the spicules to form 

 fibres, and of the fibres at large to form the skeleton ; and 

 thirdly, the spongin which binds the fibres together. 



(a) The Spicules. — In order to facilitate description the 

 spicules will be divided into three classes, the ordinary division 

 into " megascleres " and " microscleres " being intentionally 

 avoided, because it is — to say the least — doubtful whether the 

 small smooth spicules are microscleres or young megascleres. 



The three classes of spicules are — 



(a) Diactinal raonaxons which taper to a sharp point either 

 gradually (amphioxea), or more rapidly (amphitornota), and 

 are without swellings on their shaft. The former are always 

 straight, the latter curved (PI. 37, fig. 3). 



(j3) Similar straight amphioxea or curved amphitornota with 

 distinct swellings on the shaft (PI. 37, fig. 4). 



(y) Irregular systems formed by the fusion of spicules 

 belonging to class a (PI. 37, fig. 2).^ 



(a) The straight amphioxea taper gradually into a sharp- 

 pointed end (PI. 37, fig. 3, a and b), while the curved am- 

 phitornota, which are far more numerous, taper much more 

 abruptly into a similar point (PL 37, fig. 3, c — e). Both the 

 straight amphioxea and the curved amphitornota are highly 

 variable in thickness, and exhibit all stages of development. 

 The axial thread is of even thickness throughout its whole 

 length in all these spicules. 



(j3) In addition to being slightly more slender than the 

 Cf. Schulze and Lendenfeld's nomenclature (10). 



