36 SPOXGID.E. 



their possessor from the attacks of other animals. 

 Those which occur in the sarcode are usually of 

 the kind which have been denominated ' stellate,' 

 many of them presenting a curious complexity of 

 structure. Other spicules, whose forms are no less 

 peculiar, are found in connection with the so-called 

 ' gemmules ' and similai* reproductive bodies : thus 

 in Spongilla, each of the spicula by which the 

 " seed-like body " is surrounded presents the ap- 

 pearance of a pair of toothed wheels united to- 

 gether by an axle {fig. 8). 



The composition of the spicula is, in most cases, 

 siliceous, but in Gvant'ia and a few other forms 

 they are found to be composed of carbonate of 

 lime. 



It is evident from the peculiarity and constancy 

 of the forms which spicula assume, and from the 

 total absence of anything which can be compared 

 to a crystalline structure, that they are to be re- 

 garded as true organic deposits, resulting, it would 

 seem, from the vital endowment of sec^ments of 

 the sarcode body especially set apart for their se- 

 cretion. 



7. Clasf^ilication. — We possess no good clas- 

 sification of the Sponges, the several " genera " and 

 " species " into which this group of animals is 

 usually divided having been far too insufficiently 

 examined to permit any arrangement of them 

 w^hich has hitherto been proposed to be regarded 

 as aught else than temporary. We shall therefore 

 select as an example of this section of the Proto- 

 zoa the form denominated Tethya, not that it is 

 to be regarded as its most typical representative, 

 but rather because its structure has been more 



