THOBECTA. 143 



which the pit-like depressions are situated; these are sometimes 10 millim. 

 deep and deeper. The surface is covered with a stout sand-cortex, which is 

 perforated by round holes, 0"5-0'8 millim. wide and about 1 millim. apart. 

 The cortex, which is composed of sand-grains, is O'7-l millim. thick, and covers 

 the whole of the surface in a very uniform manner. 



The sponges belonging to this species do not grow to a very large size. The 

 tallest attain a height of 300 millim. 



The colour of the living sponge is yellow-greenish or grey. The dry skeleton, 

 destitute of the sand-cortex, appears light chestnut or greyish brown ; it is 

 soft, compressible, and elastic. 



The main fibres of the supporting skeleton are 0*067 millim. {T. exemplum, 

 var. tertia) to 0*13 millim. {T. exemplu7n, var. prima) thick, and slightly knotty. 

 They are rarely {T. exemplum, var. secunda) free from bodies. Generally they 

 contain scattered sand-grains &c. The connecting-fibres are 0*04 millim. 

 (T. exemplum, var. tertia) to O'l millim. {T. exemplum, vars. marginalis and 

 prima) thick, simple or slightly branched, and free from foreign bodies. The 

 meshes are mostly square, 0-5-1-2 millim. wide. 



The superficial layers of spongin in the fibres are very clearly stratified. In 

 the axis of the connecting-fibres a pith-thread can be discerned. 



In the perforations of the sand-cortex thin sieve-like membranes are spread 

 out, in each of which fifteen to twenty-five oval inhalant pores, 0-07 millim. 

 wide, are observed. From each pore-sieve a wide canal leads downwards. 

 This opens directly into one of the 0*8 millim. wide, cylindrical, straight canals 

 which extend perpendicularly to the surface down into the interior of the 

 sponge. These large inhalant canal-stems are joined at their distal ends by 

 tangential, not very wide canals, which undermine the sand-cortex. From the 

 large perpendicular inhalant canal-stems small branches are given off which 

 ramify irregularly. The final ramifications of the inhalant canal-system are 

 not under 0'09 millim. wide. The ciliated chambers are 0*045 millim. 

 wide, regularly spherical, and destitute of special efferent canals. They open 

 direct into the final ramifications of the exhalant canal-system, which are 0*04- 

 0*06 millim. wide. The larger exhalant canal-systems are pervaded by fine 

 membranes, which partly divide them into separate compartments. 



The wider oscular tubes are generally smooth and destitute of sphincter- 

 membranes. 



Thorecta exemplum, var. prima, nov. 

 Lamellar, erect, curved sponges, which may attain a more or less cup-shaped 

 form by the approximation of the lateral, margins of the curved lamella. The 

 sponge attains a height of 150 millim. and a breadth of 100 millim. The 

 lamella is about 10 millim. thick. The free margin is pretty sharp, the lamella 

 thinning out towards it. 



