Sponges. 55 



into the outflowing current from the large holes (oscula). At the junc- 

 tion between the two systems of tubes are the most vital organs of the 

 sponge , little swollen cavities of microscopic size walled in with tiny 

 living particles , each bearing a vibrating hair , which it lashes on the 

 current , and a transparent filmy skirt , with which it catches any food 

 that may pass. 



All this labyrinth of canals and cavities is living , soft flesh. To 

 prevent it falling a ready prey to the first hungry animal that passes, it 

 is set through and through with little fiinty needles or thorns. A smaller 

 group of sponges has its spines of chalk, to servo the same end. A very 

 large number of the flinty sponges cement their spines together with the 

 horny substance already referred to ; a few have lost the flinty spicules 

 entirely, and, to withstand better the shocks of the waves, have replaced 

 them by the more elastic cement. The net-like skeletons of this last 

 small group form the sponges — bath-sponges , toilet-sponges , and the 

 rest , with which we habitually associate the name. The animals in 

 which they Avere contained are killed by exposure to the air, and then 

 removed by repeated washing. 



The chalky sponges {Calcarea) are mostly small, and either grey 

 or white. A fair type is the Sycon raphanus , of which a variety pecu- 

 liar to the Aquarium grows thickly on the walls of its tanks (Fig. 159). 

 The flinty sponges ( Silicea ) are the most 

 numerous and varied ; to these belong in the 

 Aquarium the orange-coloured branches of 

 Axinella (Fig. 120), the yellow balls of Tethya 

 (Fig. 119) ; also the apparently free-moving 

 Suberites (tank 23, see p. 73) under which, 

 hoAvever , Avill be found in such case a crab 

 using it as a protective covering. 



The sponge of commerce is of the form 



Ave know in its domestic relations , but in life 



shoAvs on its surface the largest onl}" of its 



numerous holes (Fig.ll8);over all the rest the dark, 



slate-coloured flesh forms a continuous film. t-- .rr. ^ 



Fig. 159. .Some speciiueiis 



It is obtained by diving, dredging, or harpoon- o( Sycon raphanus. attached 

 ing with a long trident ; the principal markets ^^ ^ P''^[" "^ ''"'^^ ^^ '^^"^ 

 are at Trieste and Paris. Of the ditt'erent kinds 



the finest and most costly is the Levantine sponge {Empongia offici- 

 nalis) which , in its varieties , extends on all the eastern Adriatic and 

 Mediterranean shores. It is not found west of Naples, and on the Italian 

 coast no sponges occur in remunerative quantity, though quite recently 

 some beds have been discovered near Sicily. The harder Zimocca sponge 

 {Euspongia zimocca), from Asia Minor and Egypt, fetches about one 

 tenth of the price, as does the large coarse horse sponge (Hippospongia 

 equina), found in all the Levant and extending along Africa to the Straits 

 of Gibraltar ; it is honey-combed Avith Avide holes. Of the last genus 

 ( Hippospongia ) are the American « glove sponge » and « sheeps-AVOol 

 sponge »; their « hard-head » is related to the European Zimocca, while their 

 « veh-et sponge » and « grass sponge » are independent species. The Bahamas 



