53 



The terminal sieve-plate is on the whole vertical to the axis of the sponge 

 and strongly convex. It consists of a very irregular network of thin strands, 

 thickened at the nodes, which enclose triangular to hexagonal meshes, 1-2 mm. 

 wide. 



The better the specimens are preserved the easier it is to make out the 

 presence of a second, basal sieve-plate, similar to the one described by Marshall 

 in Euplectella oweni. This basal sieve-plate has the appearance of a transverse, 

 septum-like continuation of the lateral tube-walls and is also bent outwards, 

 convex towards the base of attachment. Below the basal sieve-plate lies the 

 free root-tuft, which is destitute of soft parts. 



The main supporting skeleton consists of a tube-shaped network of spi- 

 cular fibres, arranged strictly longitudinally and transversely and intersecting 

 each other at right angles. In the central part of the body of larger specimens 

 these are 3-6 mm. apart and up to 500 h- thick. The longitudinal fibres lie 

 nearer to the outer, the transverse fibres nearer to the inner surface of the tube- 

 wall and the latter protrude into the gastral cavity like internal hoops. The 

 longitudinal fibres approach each other in the lower considerably attenuated part 

 of the tubular sponge, and slightly also in the upper less attenuated part, which 

 is at the top for some distance of uniform thickness and regularly cylindrical. 

 The transverse fibres, which form the internal hoops, are throughout the sponge 

 uniformly 3 mm. apart. Besides these longitudinal and transverse fibres there 

 also exist thin fibres crossing the longitudinal and transverse ones under an 

 angle of 45°. These are entwined with the latter and form two systems of 

 spirals intersecting each other vertically. They are so arranged as to cut off 

 the corners of the square meshes in the primary network composed of the 

 longitudinal and transverse fibres and give to them an octagonal shape, as has 

 first been clearly discerned in Euplectella oweni by W. Marshall.* 



All the meshes of this parietal network are free from spicular fibres, and only 

 covered by soft tissue, which can easily be perforated by indifferent parietal 

 apertures. In Euplectella asp)ergillmn and other species, the meshes of this 

 primary network traversed or covered by spicular fibre, alternate with meshes 

 not thus covered, both in the longitudinal and in the transverse direction. 

 As the parietal apertures are restricted to the meshes not covered by spiral 

 fibre, they are in these Euplectellids not situated side by side but are arranged 

 like the black fields of a chessboard in oblique or diagonal, more correctly 

 speaking, in spiral lines. 



The spicules which compose this strong and elastic skeleton-net are in the 

 older specimens to some extent glued together by silica. This concrescence is 

 not carried so far however as in Euplectella aspergillum, where it leads to the 



* " Untersuchungen uber Hexactinelliden "' in Zeitschr. wise. Zool. Bd XXV, Suppl. 



