HALME. 161 



The canal-system is rather complicated. The inhalant pores are scattered 

 all over the surface, on an average 0-2 millim. apart ; they are circular, oval, 

 and apparently very liable to change in shape and size ; it seems that they can 

 be dilated and contracted at the option of the sponge very considerably. Their 

 average width is about 0-04 millim. They are covered outside by a fine 

 and tender sieve-membrane, with numerous, about 20, circular pores. From 

 the inhalant pores short, circular, and cylindrical canals, about 0*04 millim, 

 long and as wide as the pores themselves, lead down into the subdermal cavity. 

 The latter is not very highly developed, and consists of a system of wide, anas- 

 tomosing, radially depressed canals extending tangentially and undermining 

 the skin. The outer side is flat, and lies in a plane parallel to the outer surface. 

 The floor is very irregular, forming wide, conic extensions which lead into the 

 inhalant canals. The average width of the subdermal cavity is only 0-03 

 millim. The inhalant canals are much curved and only slightly ramified, more 

 or less circular in transverse section, and average about 0-18 millim. in diameter ; 

 they are accordingly much wider than the subdermal cavity. Their ramifica- 

 tions are irregular. The exhalant canals are much more irregular in shape 

 than the inhalant ones, and of similar average width. Also these are slightly 

 and irregularly ramified, and join to form lacunose cavities — the oscular tubes. 

 The average width of these tubes is 0-6-0-8 millim, ; they are constricted just 

 below the circular oscula, which are not raised over the surface and appear 

 scattered irregularly. The width of the osculum is 0-12 millim. The ciliated 

 chambers are spherical, and 0-04 millim. wide; they form a dense layer 

 taking up the whole of the thickness of the lamellae, which divide the inhalant 

 from the exhalant canals. 



The skeleton consists of a pretty regular network of fibres which do not 

 contain any foreign bodies. Main and connecting-fibres cannot be distinguished. 

 The fibres have a thickness of 0-006-0-02 millim., and form a pretty loose net- 

 work. In the true body of the sponge the meshes average 0-2 milKm., and in 

 the vestibule-tissue 0-5 millim. in width. At the joining-points of the fibres, 

 in the true body of the sponge, only sand-granules 0*14 millim. in size are 

 situated, one in each joining-point. These sand-grains, which form an integral 

 part of the skeleton, are enclosed in a spongiu coating, which is about half as 

 thick as the fibres ; it is stratified, but the layers are not very clearly visible. 

 There can be no doubt that these sand-grains are_ originally attached to the 

 tips of the conuli, and from thence apparently wander centripetally, as they 

 actually remain in the same place whilst the sponge grows, and the conuli 

 extend beyond them. 



This species is interesting for the development of the nervous elements which 

 I have discovered in the membranes pervading the vestibular lacunae. 



M 



