Sfri'duir (Old Dcvelopmenf of a Horny Spoiir/c. G3 



distinguislied by tlie presence in tliein of nimieious foreign 

 bodies, which form an axial core, surronnded and lield 

 together by the s])ongin. The secondary fibres are short and 

 contain no foreign bodies, they run in various planes and 

 unite together adjacent primary fibres, meeting them at 

 various angles ; they may also branch and anastomose 

 infer .sr. 



Thus the skeleton is thoroughly typical in structure and 

 arrangement, and essentiall}' the same as that of the ordinary 

 bath sponge, only much coarser. 



In the stalk, the skeleton is more strongly developed, and 

 hence the latter acquires a tougher and denser character than 

 the remainder of the sponge. 



(c) CdiKil Si/stem. — The inhalant apertures, or pores, are 

 thickly scattered all over the depressed areas on the surface 

 of the sponge. These larger areas are themselves subdivided 

 into a great number of smaller ones, which are the true 

 l)ore-areas, comparable to the pore-sieves of Phakellia 

 and MyxiUu. Each poie-area is irregularly rounded or oval 

 in outline, and measures about 019 mm. in its longer 

 diameter. Each one overlies a subdermal cavity, and contains 

 five or six oval or rounded pores, averaging O'Oo mm. in 

 diameter. 



The subdermal cavities are hollow spaces in the ectosome, 

 corresponding in size and shape to the poie-areasAvhich they 

 underlie, and communicating with the exterior by means of 

 the pores in the latter. Adjacent subdermal cavities are 

 separated from one another by anastomosing veitical walls of 

 tissue, constituting the bulk of the ectosome, and each 

 coiiimunicates below Avith an inhalant space or canal. 

 Thus each subdermal cavity receives the water from the 

 exterior through five or six distinct apertures (the pores) in its 

 roof, and passes it on thrt)ugh a single (?) ajierture in its floor 

 into a large inhalant channel. Just as a number of pores 

 lead into one and the same subdermal cavity, so also a 

 number of subdermal cavities lead into one and the same 

 inhalant channel. There is, however, a good deal of variation 

 in the arrangement of the inhalant canal system, and it 

 Avould probably be difticult to find two cases in which it was 

 exactly alike. The large inhalant channels, into which the 

 subdermal cavities directly open, lead, in their turn, into an 

 irregular system of much smallei', more or less lacunar 

 channels, whose ultimate i-amifications open into the flagellated 

 chambers. 



