HYDR,E. SPONGE. 



been turned inside out, 

 and yet the cavity thus 

 formed, having the skin 

 inside, performed the 

 functions of the natural 

 stomach ; but what is 

 most singular and as- 

 tonishing is their great 

 tenacity of life, which 

 enables them to live 

 even after they are cut 

 into pieces, and each 

 fragment afterwards be- 

 comes an entire and per- 

 fect hydra. 



Fig. 90. HYDR.E. 



" When left free, the hydrae are found to select positions most exposed In 

 the influence of light, assembling at the surface of the ponds which they 

 inhabit, or seeking that side of the glass in which they are confined, that ia 

 most strongly illuminated. That they are able to appreciate the presence 

 of light is therefore indubitable ; yet with what organs do they perceive it ? 

 we are driven to the supposition, that, in this case, the sense of touch sup- 

 plies to a certain extent the want of other senses, and that the hydrae are 

 able to feel the light. 



" When the hydra is watching for its prey, it remains expanded (Jig. 90, 

 6), its tentacles widely spread and perfectly motionless, waiting patiently 

 till some of the countless beings which populate the stagnant waters it fre- 

 quents, are brought by accident in contact with them : no sooner does an 

 animal touch one of the filaments, than its course is arrested, as if by 

 magic ; it appears instantly fixed to the almost invisible thread, and in spite 

 of its utmost efforts is unable to escape ; the tentacle then slowly contracts, 

 and others are brought in contact with the struggling prey, which, thus 

 seized, is gradually dragged towards the orifice of the mouth, that opens to 

 receive it, and slowly forced into the interior of the stomach." Jones. 



21. SPONGES live in the sea, attahed to rocks: they bear 

 some analogy to the common mass in which certain polyps are 

 lodged, but we find none of these ani- 

 mals on them. Their surface is per- 

 r orated by an immense number of 

 holes which communicate with canals 

 running through their substance in 

 every direction, and through which 

 currents of water are continually 

 passing (fg 91). Sponges are found 

 in a variety of forms; some are like Fig. 91. SPONGE. 



Explanation of Fig. 90. a, represents small patches of vegetable mat 

 ter, floating on the water, beneath which hydrtE are ordinarily found; 6 

 jne of these polyps ; c, another, having two young ones attached to it. 



21. What are sponges ? Where are they found ? 



