136 HIPPOBPONGIA. 



the surface. The meshes of the network are 0-16 millim. wide, and the raised 

 lines 0'08 millim. broad. The latter consist of sand-grains, which measure 

 from 0'15-0-28 millim. In the meshes of this network the pore-sieves, each 

 with 8-20 small pores, are situated. Corresponding to the grooves in the 

 skeleton above described, there are askeletous portions of the sponge, mostly 

 disposed in longitudinal lines, the surfaces of which are destitute of couuli, and 

 often moi'e or less retracted or collapsed ; the small circular oscula, 1-2 millim, 

 wide, are confined to these areas ; they are disposed in lines, and on an 

 average 10-15 millim. apart. 



Circular vents are situated on the summits of the digitate processes, which 

 closely resemble oscula, but are in reality the entrances to the system of vesti- 

 bular lacunae — the large cavities in the digitate processes. These lacunae have a 

 circular transverse section and measure 5 to 12 millim. in width ; they are mostly 

 upright, vertical, and here and there form anastomoses. About one half to two 

 thirds of the whole sponge is occupied by these vestibular spaces, which are 

 conoiected with the outer world only by the pseudoscula on the summits of the 

 processes. No oscula open into these cavities, theu' walls are perforated only 

 by inhalant pores, and so they appear as an outward appendage to the inha- 

 lant system — a vestibule. 



The inhalant pores can be entirely closed at the option of the sponge. la 

 consequence of this, the number in each pore-sieve found open and visible is 

 very variable. The greatest number counted by me in one pore-sieve was 20. 

 The pores are circular, and uniformly scattered over the whole of the pore- 

 sieve. When fully dilated they measure O'Ol millim. in diameter. The pores 

 on the inner surface open into the vestibule, those of the outer surface into the 

 outer world. Below the pore-sieves extensive subdermal cavities are met with ; 

 these are larger below the inner surface than below the outer. The subdermal 

 cavities below the inner vestibular surface are composed of irregular, tangential, 

 and mostly longitudinal canals with an irregularly oval transverse section, which 

 may attain 3 millim. in their largest diameter. Such immense subdermal cavities 

 are rare. The subdermal cavities below the outer surface consist of similar 

 irregular longitudinal canals, which, however, are very much narrower and 

 more radially compressed, attaining a width of 1 and a height of 0-5 millim. 

 Canals extend from these extensive cavities down towai'ds the interior of 

 the sponge; these have an average diameter of 0-1 millim., a circular or oval 

 transverse section, and extend in a more or less longitudinal direction. They 

 ramify continually, and the smallest final ramifications, which have the same 

 shape as the larger canals, measure 0-02 millim. in width. The ciliated 

 chambers form three fourths of spheres and measure 0-032 millim. in diameter. 

 The exhalant canals are similar in shape, size, and direction to the inhalant 

 ones, and join to form larger stems, which do not extend longitudinally, but 

 curve towards the askeletous portions of the sponge, obtaining a more and 



