Siliceous S^ncules of Sponges. 205 



think It better to write a short essay on the subject, referring 

 to the figures that are at present accessible in the works of 

 Bowerbank, Schultze, Oscar Schmidt, Carter, and others. 



The spicules are sometimes entirely absent in some horny 

 fibrous sponges ; and at others there are only a few, sunk in 

 the substance or in the fibres ; but these become gradually more 

 abundant until the sponge seems formed of groups of spi- 

 cules only kept together by a very small quantity of animal 

 matter. Others are scattered on the external surface of the 

 horny skeleton. 



They are generally quite separate from one another ; but in 

 some sponges, which have been called Coralloid Sponges, the 

 siliceous matter secreted by the animal is so abundant that it is 

 deposited round the spicules, more or less uniting them together 

 by a hard siliceous coat ; but the spicules ought to be described 

 in their separate state, in which they can almost always be seen 

 through the siliceous matter by which they are agglutinated 

 together. Such agglutinated spicules are figured in the * Bri- 

 tish Sponges,' t. XV. f. 274, 275, 276, 277, 278, and also in 

 Schmidt's ' Atlantic Sponges,' t. i., ii., & iii. 



In observing the spicules of sponges, one should look out 

 for the normal forms, and carefully abstain from being mis- 

 led in the description of the species by the observation of the 

 undeveloped or irregular forms which they sometimes assume, 

 though it may be very interesting to trace these accidental 

 forms to the usual state of the spicule. Dr. Oscar Schmidt, 

 in his work on the Sponges of the Adriatic and Algiers, seems 

 to delight in figuring the abnormal forms and the agglutinated 

 spicules, instead of the regular typical forms which give the 

 real characters of the species. 



The siliceous spicules may be divided into several types : — 



1. Needle-lihe, subcylindrical spicules, sometimes tuber- 



cular. 



2. Hamate spicules, which are unilateral, curved at each end, 



subcylindrical, or more or less expanded. 



3. Quinqueradiate spicules, which have three rays diver- 



ging from a central point, from the sides of the more or 

 less elongated axis. 



4. Sexradiate spicules, with four equidistant rays diver- 



ging from a central point from the sides of the more or 

 less elongated axis. 



5. Midtiradiate or stellate spicules, with five or more rays 



arising and diverging from a common centre. 



G. Spicular spherules, formed of a multitude of needle-like 

 spicules diverging from a centre, forming a solid glo- 

 bular or oblong mass, with an areolated surface. 



Ann. cO Maq. N. IHst. Ser. 4. Vol. xli. 15 



