398 EVENINGS AT THE MICROSCOPE 



Though the substance of the polype be soft and flexi- 

 ble, it contains solid elements. Just below the expansion 

 of the tentacular blossom we see imbedded in the skin a 

 vast aggregation of calcareous spicula. Individually, these 

 are very minute, and their form is swollen in the middle, 

 and taper at each extremity, the whole roughened with 

 projecting knots. Collectively, they are grouped in regu- 

 lar 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. 



Toward 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 scien- 

 tific curiosity we shall see something of its internal struct- 

 ure. When removed from the water, the blower-like poly- 

 pes 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 toward every part of the circum- 

 ference, dilating here and there to form the cells which 

 protrude and retract the polypes. This is a complete sys- 

 tem of water-supply: the surrounding sea- water, entering 

 at the mouths of the several polypes, bathes the whole 

 interior, and conveys oxygen and the products of diges- 

 tion together to every part of the compound organism. 



The fleshy substance which surrounds these canals is 

 of a loose spongy character, and grates beneath the knife; 

 a circumstance which is owing to the predominance of the 



