74 MANUAL OF ZOOLOGY. 



they are constant for any given species, in any given part of 

 its organisation. The sponge-flesh is much more uniform in 

 its nature and composition. It may be noticed, however, that 

 in Spongilla the sponge-particles are filled with green granules, 

 which are apparently identical in chemical composition with 

 the green colouring matter of plants (chlorophyll). In Grantia, 

 too, the sarcoids are furnished with long filamentous appen- 

 dages or cilia (fig. 3, d\ The siliceous sponges are mostly in- 

 habitants of the deep sea, and many of them are remarkable 

 for their long and slender spicules of flint. In Hyalonema^ or 

 the glass-rope, long placed amongst the Zoophytes (Zoantharia 

 sclerobasica), and still considered as such by competent authori- 

 ties, there is a cup-shaped sponge-body, supported by a rope 

 of long twisted siliceous fibres, which are sunk in the mud 

 of the sea-bottom. In other " anchoring sponges," such as 

 Pheronema and Holtenia, the body is sessile or stemless, and is 

 moored to the mud by a beard of long delicate spicules. These 

 sponges, in their single, long, chimney-like osculum, show a 

 curious resemblance to the fossil Siphonias of the green sand. 

 Similar root-fibres of flint, traversing the mud in every direc- 

 tion, occur in the beautiful Venus' Flower-basket (Enplectelld), 

 without any exception one of the most exquisite of all organic 

 structures known to us. 



DISTRIBUTION OF SPONGES IN SPACE. Sponges are almost 

 exclusively marine, the SpongUl& alone being inhabitants of 

 fresh water ; and they are of almost universal occurrence. The 

 sponges of commerce are mostly obtained from the Grecian 

 Archipelago and the Bahama Islands. Recently the existence 

 of numerous siliceous sponges at great depths in the ocean has 

 been demonstrated by Drs Carpenter and Wyville Thomson. 

 They are associated with numerous Foraminifera and with 

 Crinoidea, the whole assemblage bearing a singularly close re- 

 semblance to the fauna of the Cretaceous epoch. The com- 

 mon marine sponges are mostly found attached to some solid 

 object between tide-marks or in deep water. The vitreous or 

 siliceous sponges appear to be exclusively inhabitants 'of the 

 deeper parts of the ocean. One genus (Cliona) inhabits 

 branching cavities in shells, which the sponge excavates for 

 itself apparently by means of its siliceous spicula. Fossil 

 shells mined by a boring-sponge, allied to the recent Cliana, 

 are found from the Silurian rocks upwards. 



DISTRIBUTION OF SPONGES IN TIME. Remains of sponges 

 are known to occur in formations belonging to the Palaeozoic, 

 Mesozoic, and Kainozoic epochs. The keratose or horny 

 sponges are obviously incapable of leaving any evidence of 



