HIPPOSPONGIA. 137 



more transverse direction the larger they become. The askeletous. portions of 

 the sponge are much less dense than other parts, and occupied by a very loose 

 lacunose tissue. These lacunaB communicate and open out into the short and 

 wide oscular tube ; they are on an average 0-6 millim. wide, and the membranes 

 which separate them are in some places only 0-005 millim. thick. In portions 

 also of this askeletous part, ciliated chambers are found, particularly in the 

 depth ; no chambers open directly into the large oscular tube, which measures 

 1-2 millim. in diameter. The skeleton is often interrupted throughout the 

 entire thickness of the sponge-lamella between the vestibular space and the 

 outer surface. In such cases the lacunose tissue formed by the congregation 

 of the exhalant canals extends down to the vestibule. 



The main fibres of the skeleton extend from the point of attachment outward 

 and upward, mainly in a longitudinal direction, and branch in a more or less 

 penicillate manner. They are distally joined to the arenaceous cortex, and it 

 is clearly visible that the sand-core of the main fibres is a direct continuation 

 of the sand in the skin. The main fibres curve in their distal portions grace- 

 fully outward, and all their ends abut on the outer surface. The main fibres 

 grow just below the outer skin. It is very remarkable that no main fibres 

 are joined to the inner surface — the cortex — on the vestibular side. The 

 consequence of this is that the cortex on the inner side can readily be detached 

 whilst the cortex on the outer side appears firmly attached. In detail the 

 main fibres, although always completely filled with foreign bodies, differ in the 

 difFei-ent varieties. The connecting-fibres form two systems. Thicker primary 

 tangential, and thinner secondary radial connecting-fibres are usually distin- 

 guished. The mode of ramification varies in the varieties. The connecting- 

 fibres never contain foreign bodies. 



The skeletons part of the sponge is divided from the askeletous portion by a 

 membrane, which extends down from the outer surface to the bottom of the 

 groove. This membrane surrounds the part of the sponge occupied by the lar^e 

 exhalant lacuna ; it consists of radially disposetl, parallel, elongate, spindle- 

 shaped cells, which measure 0-032 millim. in length and in their centre 0-0015 

 millim. in thickness. The ends of these cells are very slender, forming 

 extremely fine threads. The elongate oval nucleus is situated in the middle of 

 the length, but not in the axis of the cell; it lies near the side and measures 

 0-0034 millim. in length and 0-0011 millim. in width. The cell is entirely 

 filled with very granular and highly colourable protoplasm. The granules 

 are remarkable for their large size, which may be estimated at 0-00025 millim. 

 The ground-substance in which these granules are imbedded does not refract 

 the light very much ; the granules, however, are highly refracting. The 

 ground-substance refracts the light simply, whilst the granules show when 

 examined with the polarizer that they refract the lujht doubly, iu a similar 

 manner as the discs iu the striated muscles of higher animals. These granules 



