LITHISTID SPONGE FROM THE DEEP SEA. 323 



arms (Plate XXIV. fig. 6) and little fixed spinules on the edge of 

 the limbs. The spaces produced by the junction of these spicula 

 are very small, and are intruded upon by the ornamentation as 

 well as, to a certain extent, by the spicular arrangement of the 

 limb-end. This more or less close structure is increased by the 

 presence of minute acute-pointed spicula seated on the limb-ends 

 (Plate XXIV. fig. 7) ; those of approximate limb-ends interlock. 

 In many instances the junction is by simple contact of smooth 

 surfaces, without the intervention of this minute spinulose element, 

 or by flattened-out disks and ragged ends. Throughout, the pecu- 

 liarity of the spicula is their general plainness and their scanty 

 ornamentation of projecting spinules, sharp or blunt-headed. 



The bases of the large spicula of the columns merge into 

 or join, by ragged junction, short, stout, and irregularly shaped 

 spicula, with a limb curved outwards towards the periphery 

 (Plate XXIV. fig. 8). These last form the groundwork of the 

 cortex, and they are crowded, the spaces formed by their junc- 

 tions being very small. Their curved outer limb is covered with 

 short and thick projections, more or less cylindrical and expand- 

 ing outwards, where they are often ragged (Plate XXIV. fig. 8). 

 These processes form a layer just within the outside of the cortex ; 

 and as they are close and their interspaces are excessively small, 

 there is an approach to solidity. 



The last kind of spicule to be noticed (Plate XXIV. fig. 9) 

 forms the outside of the sponge, and consists of a very short base, 

 which is narrow and more or less circular in transverse outline, and 

 a widely expanded part perpendicular to the base and placed with 

 its baseless face outwards. The expanded parts of these peltate 

 spicula produce the smooth glistening surface of the sponge. Their 

 shape is, moreover, very remarkable. An original trifid arrange- 

 ment can sometimes be traced, and the stumpy cylindrical stem 

 is the fourth limb ; but each of the tripartite portions is broken 

 up into a series of very ragged, deeply incised, dendritic-looking 

 processes. These branch and rebranch, and terminate in minute 

 ramuscules, ending often in minute sharp points arranged in a 

 most ragged and eccentric manner. The ramifications give a 

 dendritic appearance to these connective spicula, and, to a certain 

 extent, they interlock with those of their neighbours. The spaces 

 left between the ramifications are small and microscopic and act 

 as pores. The absence of free stems to these foliato-expando- 

 ternates has been already noticed, and hence the sponge is rigid. 



LINN. JOUBN. — ZOOLOGY, VOL. XV. 25 



