112 WORMS, MOSS-POLYPS, SPONGES, ETC. 
Other encrusting forms, but with the pores or 
cells arranged in only a single tier, are the ‘ sea- 
MEMBRANIPORA (magnified). POLYPIDE (magnified). ALCYONIDIUM. 
mats,’ which are usually found on the fronds of the 
sea-weed. 
One of these (Membranipora) makes 
small scale-like patches, while another (Flustra) 
occurs in ramifying or spreading branches. A less 
readily recognizable form of polyzoan, the Alcyo- 
nidium, making fleshy crusts a third of an inch or 
BUGULA. 
more in thickness, is frequently 
found enveloping small stones, 
twigs, ete. 
While perhaps the most distinc- 
tive polyzoans are the ones which 
make crusts like those above de- 
scribed, others grow in erect, 
branching colonies, and thus still 
more closely resemble the swaying 
fronds of the true polyps. One of 
these is the common Bugula, whose 
tree-like forms, bearing tiny cups 
on their upright branches, strongly recall the ser- 
tularians or sea-firs. They are of particular interest 
to the microscopist since they show to special ad- 
