Sponge-fauna of Norway. 137 



from which it appeared that the iris-like diaphragms extending 

 across these cavities are capable of spontaneous contraction 

 and expansion, so as to vary at will the size of the central 

 lumen ; and hence he draws the obvious inference that the 

 fusiform fibres composing these diaphragms are not only mor- 

 phologically similar to muscle-fibres, but physiologically as 

 well ; from this he proceeds to the conclusion that the fibres 

 of Tethya and other rind-sponges are likewise muscle-fibres. 



Hackel* does not deny that the fusiform fibres are both irri- 

 table and contractile, in the sense of shortening in the long and 

 broadening in the transverse direction ; but he maintains that 

 true muscle cannot be evolved without a simultaneous dif- 

 ferentiation of nerve-tracts ; and since specialized nerve-tracts 

 do not exist in sponges, he would call the contractile fibres in 

 question " neuro-muscles." 



Carter ~\ describes the fusiform cells, referring to his figures 

 in the Ann. & Mag. Nat. Hist. 1872, vol. x. pi. vii. figs. 

 9, 10, in illustration. These cells are less specialized than 

 those to be met with in many other instances (they resemble 

 fusiform connective-tissue corpuscles) ; but Carter decides to 

 regard them provisionally as muscular. 



F. E. Schulze\ figures and describes fusiform cells also 

 from an Aplysina (A. acropkoba) ; he follows Hackel in 

 refusing to designate them as muscle-fibres, preferring the 

 term " contractile fibre-cells." 



Carter §, in his account of Axos spinispiculum, Carter, 

 describes some fibrillated fibres which he conjectures may be 

 muscular, especially as they lie parallel to each other and are 

 not united as in elastic tissue. 



In Stelletta Normani the fibres are the best marked I have 

 yet met with in any sponge, and they likewise most closely 

 resemble the organic muscle-fibres of the higher animals ; 

 they are about 0-0066 inch long and 0'0003 broad, fusiform, 

 hyaline, colourless, and of sharply marked contour ; their 

 nucleus or axial thread, as it may be more correctly termed, 

 is fusiform, homogeneous, faintly bluish in colour, highly 

 refringent, and 0-0035 inch long (PI. VII. fig. 20). With 

 polarized light the fibres behave like uniaxal crystals. 

 Treated with acetic acid or boiled in water they undergo no 

 appreciable change ; but potash and nitric acid produce well- 



* Hackel, 1872, Die Kalksehwamme, p. 414. 

 t Carter, 1875, Ann. & Mag. Nat. Hist. ser. 4, vol. xvi. p. 36. 

 X Schulze, 1878, Zeitsckrift f. wiss. Zool. p. 394, pi. xxii. fig. 13. 

 § Carter, 1879, Ann. & Mag. Nat. Hist. ser. 5, vol. iii. pp. 287 and 290. 

 pi. xxv. tigs. 6-8. 



