408 On the Sponge-fauna of Norway. 



exactly applied to the ostium of the incurrent chone of the 

 Isops ; and thus the supply of food and water brought to this 

 particular chone can only reach it after straining through the 

 intercepting parasite. 



Should the association, which we may here regard as acci- 

 dental, become permanent, great structural changes would 

 probably be produced in the parasite : for one thing, the 

 collared cells would be relieved of the necessity of propelling- 

 water through the organism, and could restrict themselves to 

 gathering food for it ; and no doubt this would lead to various 

 other modifications. That in certain cases the association does 

 become persistent is quite certain ; for in a large specimen of 

 Ectyon sparsus contained in the Bristol Museum we find a 

 large number of the oscules lined each by a small parasitic 

 Geodia belonging to an undescribed species ; but as this 

 specimen is unfortunately not preserved in spirits, we cannot 

 determine the kind of histological change which may have 

 been induced in it. 



EXPLANATION OF PLATE XVII. 



Isops Vhletjrcei (n. gen. et sp.). 



Fig. 1. Section across the rind, showing an excurrent chone (E) and an 

 incurrent chone with a young Oeodine sponge (p) grown over 

 its mouth : f, subcortical hbrous layer; c, gelatinous connective 

 tissue (X 15). 



Fig. 2. A long-shafted spicule enveloped in the spicule-sheath, s, and 

 encircled by single, fusiform, hyaline fibres, g ( X 14-0). 



Fig, 3. Transverse section of the rind, showing incurrent chone with its 

 sphincter protruding into the subjacent crypt ( X 15). Canada- 

 balsam preparation. 



Fig. 4. Transverse section of lower part of rind : g, lowest-lying glo- 

 bates of globate layer; f, fibrous layer; c, gelatinous connective 

 tissue with scattered fusiform Hbres and an oval granular cell ; 

 r, rugae of cryptal canal (x 70). 



Fig. 5. Fusiform acerate spicule ( X 15). 



Fig. <>. Head of slender ternate spicide with recurved rays (x 00). 



Fig. 7. Tritid spicule (x 15). 



Fig. 8. Head of slender porrecto-ternate spicule, with only one ray deve- 

 loped ( x 60). 



Fig. 9. Bifurcated tritid spicule (X 15). 



Fig. 10. Large stellate from the mark (x 435). 



Fig. 11. Section across the dermal layer : e, epidermis ; r/, dermis, with 

 intercalated stellates; g, outermost globates of the globate layer 

 (X 217). 



Fig. 12. Splnero-stellate of the rind (X 485). 



Fig. 13. Stellate of the mark, usual size (X 435). 



Fig. 14. Outline of globate ( X 60). 



Fig. 15. A granular cell with terminal filament; from the gelatinous con- 

 nective tissue of the subcortical layer ( X 435). 



Tin. 16. Similar to tig. 15, but without the extended filament (X 435). 



