MANUAL OF NATURAL HISTORY. 375 



prepared by soaking in dilute hydrochloric acid, in 

 order to remove all traces of lime, and then bleached 

 and well-beaten, to render them fit for the market ; 

 but the greater majority of Sponges are unfit for 

 useful purposes, on account of the large amount of 

 siliceous and calcareous spicula with which their 

 skeletons are strengthened. Although the portions 

 of a Sponge, when cut in half, will each grow and 

 become perfect sponges, yet their usual mode of pro- 

 pagation is by means of gemmules, which sprout 

 from the delicate granular film covering their ske- 

 leton; and which, gradually becoming detached 

 from the parent body, are furnished with cilia, swim 

 freely about, and afterwards, finding a favourable 

 locality, become fixed, and gradually enlarge into 

 Sponges. 



III. CLASS. SPONGES (Porifera). 



Body multiform, fixed, fleshy, composed of a 

 fibrous axis, covered with a gelatinous coating; 

 often interwoven with siliceous or calcareous spicula. 

 Reproduction by gemmules. 



I. ORDER. HORNY-SPONGES (Cornea). 



Body very porous and elastic, soft, composed of a 

 fibro-corneous skeleton, which anastomoses in all 

 directions ; without any spicula. 

 1. FAMILY. Sponges-proper (Spongiidae). Body 

 irregular, traversed by tortuous canals, open- 

 ing externally by distinct oscula. 



