158 ANATOMY AND PHYSIOLOGY 



Halichondria, and in H.panicea, it assumes very striking 

 proportions in excessively developed specimens, whilst in 

 others it is either an occasional, uncertain, and progressive 

 organ, or is altogether absent. 



The mode of propagation is also an uncertain character. 

 Thus in Tethea cranium we find it to be by internal gem- 

 mulation ; in T. lyncurimi by external gemmules ; and in 

 other species of the genus no gemmules of any description 

 have hitherto been detected. In Geodia, Pachymatisma, 

 and Spongilla the general structure and mode of disposition 

 of the ovaria render them valuable auxiliary generic cha- 

 racters, but in other cases they are of little or no value. 



The intermarginal cavities are available as generic cha- 

 racters in Geodia and the nearly allied species, and in the 

 same sponge the relative position of the connecting spicula 

 form good distinctive characters in the genera Geodia, 

 Ecionemia, and also some of the siliceo-fibrous sponges. 

 In Alcyoncellum., Polymastia, and Geodia the position and 

 appendages of the oscula are also available, but generally 

 speaking those organs are so mutable as to render them of 

 little value as generic characters. 



The following tabular view of the arrangement I propose 

 to adopt, will perhaps render the details regarding the dis- 

 tinctive characters and natural affinities of the genera more 

 readily comprehensible. 



