WORMS, MOSS-POLYPS, SPONOES, ETC. m 



quently be seen on the insides of shells, around 

 pebbles, over the tubes of the serpula, etc. From 

 each of the tiny pits, which are often no larger 

 than a pin-point, protruded during the life of the 

 compound colony a minute polypide, delicately 

 crowned with tentacles, in many ways reminding 

 one of the polyps of the hydroid colonies with 

 which we have already become acquainted. But 

 these so-called moss-polyps or lace-corals — the 

 Polyzoa of naturalists — show a considerable ad- 

 vance in structure over their hydroid brethren, in- 

 asmuch as they are provided with a distinct ali- 

 mentary tract — stomach, intestine, etc. — clearly 

 marked off fi'ora the rest of the body, a central 

 nerve-mass, and other specialized organs which are 

 wholly wanting in the other group. The mouth is 

 situated at one side of, or within, the crown of ten- 

 tacles, receiving its full complement of nourishment 

 from the water-currents which are constantly being 

 impelled towards it by the vibrating 

 hairs (cilia) on the tentacles. 



Perhaps the commonest of our en- 

 crusting forms is Escharina, whose 

 delicate tracery can frequently be 

 seen interwoven in concentric layers 

 among or over the serpula tubes, 

 making rounded masses from two to 

 six inches across. In this condition they may be 

 easily mistaken for pebbles, but their comparative 

 lightness, and the numerous minute holes which 

 become visible on closer examination, soon disclose 

 their true character. 



