88 



MANUAL OF ZOOLOGY. 



in the sponges of commerce the axial hollow contains neither foreign 

 bodies nor spicules. In other cases, the central tube of the horny fibres 

 may be filled with sand-grains or with spicules, and in addition to the cen- 

 tral core the surface of the fibre may be roughened by projecting spicules ; 

 while in some cases the fibre is almost entirely made up of simple spicules 

 bound together by a small quantity of sarcode. 



In the "so-called "calcareous" sponges (Calcispongi(z) such 

 as Grantia, Sycon, &c., the skeleton is composed of numerous 

 calcareous spicules (fig. 24, A), which are in the form of simple 



Fig. 26. Fragment of the skeleton of 

 a horny Sponge (after Bowerbank), 

 greatly enlarged, showing interlacing 

 horny fibres with spicula. 



Fig. 27. Different forms of the spicules 

 of the horny, calcareous, and siliceous 

 Sponges, greatly magnified. 



fusiform rods or of three-rayed (rarely four-rayed) needles, and 

 are arranged in different ways in different species. The three- 

 rayed spicules are the form especially characteristic of the 

 Calcispongice, but two or all of the known forms of calcareous 

 spicule may occur in a single sponge. 



In the so-called siliceous sponges (Silicispongia) the skeleton 

 is composed of siliceous spicules, of various forms and vari- 

 ously disposed. Very commonly -ihe jspicules which primi- 

 tively compose the skeleton become m^process of growth fused 

 together by a secondary siliceous deposit, so that the skeleton 

 becomes a continuous one. In other cases, though the spicules 

 are permanently distinct, they are so interlocked with one 

 another as to confer practical rigidity upon the skeleton. In 

 other cases, the spicules are united with one another by sar- 

 code only. Independently, also, of the true skeleton-spicules, 

 the sarcode of the body contains scattered through it numerous 

 " flesh-spicules " of various and often very characteristic forms. 

 Modern investigations have brought to light a great number of 



