220 ANATOMY AND PHYSIOLOGY 



these characters are highly effective and valuable as specific 

 descriptions. In other genera of sponges the cloacae afford 

 striking and very effective distinction, especially in Alcyon- 

 cdlum, Polymastia, Halyphysema, and Hyalonema. Among 

 the Keratosa it also prevails to a considerable extent, but 

 in the latter order it does not afford us the same wide 

 range of striking characters that exist so abundantly in the 

 cloacse of the order Calcarea. 



10. The Sarcode. 



The universal presence and similarity in structure of the 

 sarcode of the Spongiadae renders the range of its use as a 

 specific character very limited ; but the spicula imbedded 

 in its substance so abundantly in many species are so 

 various in form, and so strikingly distinct from each other, 

 as to afford a most valuable series of discriminative 

 characters. 



The greater portion of these spicula are more or less 

 stellate in form. They vary in shape to a considerable 

 extent in each group, in consequence of incomplete or 

 complete development, and the number of the radii in the 

 stellate forms is in many cases very uncertain ; but although 

 this amount of variation exists in each of the separate 

 forms, there is always a limit to these differences, and a 

 normal character present which renders it by no means 

 difficult to decide to which class they belong. Independent 

 of the peculiar characters of their own form and modes of 

 radiation, their radii are frequently peculiarly and abun- 

 dantly spinous, and these secondary organs are equally as 

 constant and determinative in character as the primary radii. 

 The latter of these characters are frequently very minute, 

 and require the application of a high microscopic power to 

 render them available ; but they are in many cases so 

 decisively valuable, that they should never be neglected 

 when present. In truth, the spination of these and all 

 other forms of spicula are of considerable value as specific 

 characters, and their shape and direction are often indicative 



