200 SIGMATELLA. 



these sponges are pretty soft and compressible and quite inelastic ; dry, fragile 

 and friable. 



The skeleton consists of a network of fibres composed entirely of foreign 

 bodies, spicule-fragments, sand-grains, &c., cemented by a very small quan- 

 tity of spongin. These fibres are, on an avei'age, 0-2 millim. thick ; they 

 extend from the base of attachment longitudinally upward, and are joined by 

 simple unbranched fibres of equal dimensions. Thus a network with irregu- 

 larly square meshes, about 0-6 millim. wide, is formed. The surface is protected 

 by a sand-cortex 0'3-0'5 millim. thick, to which the superficial fibres of the 

 supporting skeleton are attached, often by trumpet-shaped extensions. This 

 cortex is not continuous, but consists of a network of arenaceous bands with 

 meshes 0-5-1 millim. wide; the bands themselves are about as wide as the 

 meshes which they surrovmd. In the ground-substance proper spicviles of two 

 kinds are met with : — A. Eods : these are not equally abundant in different 

 specimens, and often much more frequent in one part of the sponge than in 

 another. They are 0"17 millim. long and 0-03-0-04 millim. thick. Most 

 of them are blunt on both ends (strongyla), but styli and oxea also occur. 

 B. Sigmata : these are exceedingly small and rare, so that one often fails to 

 find any in a given specimen. I have never been able to observe them in dried 

 specimens, but have, on the other hand, always found them in the specimens 

 properly preserved by myself. Neither Carter nor Marshall, nor any of the 

 other authors, have recorded the presence of proper spicules. The sigmata 

 are only 0-005 millim. long and uniformly curved. 



Marshall gives a most extraordinary description of the structure of the canal- 

 system of his Dysidea favosa, which I believe to be identical with S. c. papillosa' 

 According to this author the inhalant pores are situated in the membranes 

 which expand in the meshes between the superficial reticulation of projecting 

 arenaceous bands. Below each of these membra nes a large cavity is supposed 

 to be situated, from which exceedingly slender canals lead down to the im- 

 mensely large sac-shaped ciliated chambers, which are also connected with each 

 other by slender transverse canals. A lateral connection of ciliated chambers 

 with each other by special canals is altogether unprecedented in sponges, and 

 my own observations by no means confirm Marshall's statement, the incorrect- 

 ness of which is probably due to the bad state of preservation of the specimens 

 at his disposal. 



According to my own observations, three to ten inhalant pores 0-02 millim. 

 wide are situated in each of the membranes which occupy the meshes of the 

 dermal reticulation. These lead into subdermal caAdties about 0-5 millim. wide, 

 which expand below these perforated plates, and are pervaded by numerous 

 trabeculse and membranes, dividing them into smaller compartments. The 

 cavities under different pore-sieves are in communication with each other by 

 large apertures. From the floor of each cavitv several inhalant canal-stems 



