OF THE SPONGIAD^. 217 



in construction from those of the skeleton, and its interior 

 surface is often supplied with anchorate retentive spicula of 

 various forms. In its sarcodous lining there are occasionally 

 an infinite number of stellate or sphero-stellate spicula to 

 protect it from the ravages of minute enemies, and its 

 surface is also often penetrated by large or small defensive 

 spicula. OccasionaUy its external surface is profusely 

 supplied with elongo-stellate defensive spicula. It has also 

 frequently a thick stratum of cellular structure of various 

 colours. 



These peculiarities of structure have no generic value. 

 They are essentially specific difierences ; and it is rarely the 

 case that any two species, even in an extensive genus, are 

 found to agree in the possession of the number, form, or mode 

 of disposition of these peculiarities of the dermal tissues. 

 They form therefore a constant and highly valuable series of 

 characters, and claim the especial attention of the student 

 in either the recognition or description of an unknown 

 species. 



5. TAe /Skeleton. 



Although the material, mode of stmcture, and arrange- 

 ment of the skeleton is more especially devoted to the 

 formation of the orders and suborders, it still presents us 

 with a sufficient number of minor peculiarities to render it 

 a source of valuable specific characters. Thas, as I have 

 already shown in treating of the relative value of the 

 spicula for the distinction of species, the difference in their 

 size afibrds a good character. The closer or more diffuse 

 mode of their arrangement modifies to a great extent the 

 form and size of the areas in spiculo-reticulated skeletons, 

 and their habitually greater or less number in the thread of 

 the reticulations produces a distinctly different aspect in 

 the skeletons of two otherwise closely allied species. The 

 presence or absence of defensive spicula, the mode of arma- 

 ture, and the forms of the defensive and other auxiliary 

 spicula also afford a very extensive and valuable series of 

 specific characters. In the kerato. and siliceo-fibrous 



