WORMS, MOSS-POLYPS, SPONGES, ETC. 111 
quently be seen on the insides of shells, around 
pebbles, over the tubes of the serpula, ete. 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 from 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, — Escuarina 
A (magnified). 
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. 
