ZOOPHYTES. 



353 



are grouped in regular forms, crowded into dense masses 

 at the foot of each tentacle ; the mass having a three- 

 pointed outline, of which the central and largest point 

 runs up into the tentacle. 



Towards the lower region of the column, spicula again 

 occur, scattered throughout the skin, and crowded into 

 groups, one on each interseptal space. These spicula are 

 of a very different shape from the upper ones; for they 

 form short thick cylinders, with each end dilated into a 

 star of five or six short branches, which are again starred 

 at their truncate ends. 



If we now sacrifice our little Cow's pap to our scientific 



SPICULA OF COWS PAP. 



curiosity, we shall see something of its internal structure. 

 When removed from the water, the flower-like polypes 

 soon retract. I now cut open the mass lengthwise with 

 a keen knife ; and you see that it is permeated by canals 

 running from the base towards every part of the circum- 

 ference, dilating here and there to form the cells which 

 protrude and retract the polypes. This is a complete 

 system of water-supply : the surrounding sea-water, en- 

 tering at the mouths of the several polypes, bathes the 

 whole interior, and conveys oxygen and the products 

 of digestion together to every part of the compound 



organism. 



*> A 



