STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 517 



it is limited externally by an epithelium of flattened cells, 

 which, however, I have not made out in my sections. The 

 most remarkable character about it, however, is the presence, 

 in its innermost portion, of two well-developed layers of fibrous 

 tissue. In the outer layer (fig. 3, 7. /.) the fibres are arranged 

 longitudinally, and in the inner layer (fig 3, c. f.) horizontally 

 or circularly. The outermost layer is not perfectly continuous, 

 but is occasionally interrupted by strands of fibres given off 

 towards the outside of the Sponge from the inner layer. In 

 short, the outer fibrous layer consists of bundles of densely 

 packed, longitudinally disposed fibres, wedged in between the 

 inner fibrous layer and the stout longitudinal skeleton-bundles 

 described above (fig. 3, /. sp.). The inner layer is about 

 0*024 mm. thick, and is composed of densely packed, circu- 

 larly arranged fibres, giving off, as already described, occasional 

 outgrowths towards the circumference. At intervals one can 

 distinctly make out much elongated, granular nuclei. In parts 

 there are distinct traces of a second layer of longitudinal fibres 

 internal to the layer of circular fibres. The histological cha- 

 racters of the different layers appear to be thoroughly identical. 

 The individual fibres are capable of separation from one another, 

 and are then seen to be very slender and greatly elongated. 

 They are, I believe, identical with the myocytes of Sollas, 1 

 and I attribute to them a contractile function. 



While speaking of the fibrous tissue, I may also notice that 

 the central oscular tube of the Sponge (fig. 2) is encased in a 

 thick sheath of fibrous tissue, continuous in the neighbourhood 

 of the osculum, with the true ectosomal layer. Here, again, 

 we distinguish between two chief layers of fibres, circular and 

 longitudinal. The circular fibres occur on the inside, i.e. next 

 to the lumen of the tube, and the longitudinal on the outside. 

 Fibrous tissue also occurs to a greater or less extent in connec- 

 tion with the main branches of the canal system. 



The Choanosome. — The term choanosome has been 

 applied by Sollas, in his ' Preliminary Report on the Tetrac- 



1 Article " Sponges," in ' Encyclopaedia Britannica,' ed. ix, p. 419, 

 fig. 21,*. 



