30 SPONGIDJ!. 



CHAPTER IV. 



SPOXGID^. 



I. Animal nature. — 2. Form and Size. — 3. Skeleton — 4. Aqui- 

 ferous system. — 5. Reparative powers. — 6. Spicula. — 7. Classi- 

 fication. — 8. Structure of Tethya. — 9. Development. — 10. Dis- 

 tribution. — II. Affinity to Foraminifera. 



I. Animal natare. — If the animal nature of 

 the Rhizopoda be admitted, that of the Sponges 

 can no longer be regarded as doubtful. For a 

 Sponge consists of a soft gelatinous substance, 

 supported by an internal framework or skeleton, 

 the whole being usually strengthened by the 

 addition of calcareous or siliceous ' spicula.' The 

 soft gelatinous flesh is found, on examination, to 

 be composed of an aggregation of amoebiform 

 bodies, and must be considered as constituting the 

 essential part of the animal, since in some Sponges 

 the skeleton is altogether absent, whilst in others 

 the mineral spicula are replaced by particles of 

 sand. 



In Grantia (one of the marine Sponges) the 

 amoeba-like particles are furnished with long 

 filamentary appendages, which have received the 

 name of ' cilia ' (^fig» 6, e). To these peculiar 

 organs, which now for the first time make their 

 appearance in the animal kingdom, we shall here- 

 after more fully allude (p. QQ\ Ciliated parti- 

 cles have likewise been noticed in the gelatinous 

 substance of the fresh-water SjDonges {SiDongilld) ; 

 but here they are not present at all times of the 

 year, having been observed to disappear on the 

 approach of mnter, during which season the body 



