176 GEODIA AMPHISTRONGYLA. 



dulating and not jierforated by larger openings visible to the unaided eye. To 

 parts of it shells, sand, and other foreign bodies are attached. It appears to be 

 covered throughout with shallow pits, the centres of which are about 1 mm. 

 apart. There is hardly a trace of a spicule-fur, but the numerous slender spicules 

 broken off at the surface, seen in sections, indicate that protruding spicules were 

 present in the living sponge. 



The colour, in spirit, is light brown on the surface and quite dark brown in 

 the interior. 



The superficial part of the body forms a cortex composed of a thin outer 

 dermal membrane, thickened in the pits; a stout central sterraster-armour 

 layer (Plate 20, figs. 33a, 39a) about 1 mm. thick; and an inner fibrous layer, 

 excavated by subcortical cavities. A thin cuticular membrane was observed on 

 parts of the surface. Whether this belongs to the sponge or whether it was 

 formed by some symbiont growing on it, I cannot state with certainty. 



Canal-system. Below the centre of each pit a radial cortical canal, penetrat- 

 ing the sterraster-armour, is situated. Above, these canals widen out to form 

 subdermal cavities which are covered by pore-sieves, the pores of wliich are 

 circular or broad-oval, 17-45 ft wide, and separated by cjuite broad strands of 

 tissue. Below, the cortical canals are restricted by chonal sphincters (Plate 

 20, fig. 33b) which protrude proximally beyond the lower limit of the sterraster- 

 armour layer. The chonal canals passing through the sphincters, which in my 

 pn'parations are strongl)' contracted and often quite closed, open out into the 

 subcortical cavities (Plate 20, fig. 33c). From the latter the choanosomal 

 canals arise. 



Skeleton. Numerous slender strands of amphistrongyles and a few styles 

 traverse the interior in a rather irregular manner. Towards the surface these 

 strands assume a radial tlirectiou and become consolidated to form bundles, 

 which ijenetrate the cortex and abut vertically on the surface. Here, in the 

 tlistal part of the choanosome and in the cortex, rhabdomes of telo-and meso- 

 clades are found in the bundles besides the amphistrongyles and styles. These 

 telo- and mesoclades are plagiotriaenes, anatriaenes, mesotriaenes, and a few 

 diaene, monaene, and other derivates of these spicules. The cladomes of some, 

 chiefly young, forms of these spicules are found in small numbers at various 

 levels. The cladomes of nearly all the adult ])lagiotriaenes and their diaene and 

 monaene derivates lie above the sterraster-armour, just below or in the dermal 

 membrane. Here also the cladomes of a few, apparently adult mesoproclades, 

 anatriaenes, and anatriaene-derivates are found. The slender spicules travers- 



