WORMS, MOSS-POLYPS, SPONGES, ETC. 109 
the habit of frequently running about or sallying 
forth has given the.name of the ‘ runners’ (Erran- 
tia). The Nereis itself is a nocturnal animal, and 
sometimes appears in large numbers swimming 
about near the surface of the ocean. 
All the forms that have here been noted have 
the body made up of a considerable number of 
rings or annuli, ranging to several hundred, which 
on either side give origin to two rows of spines 
or bristles, whence the term Cheetopoda, ‘ bristle- 
footed,’ as applied to the members of the group 
collectively. The crooked tubular habitations of 
Serpula dianthus can be seen on almost all ob- 
jects that have drifted down to 
about low-water level—on old 
pots, pebbles, the under and 
lateral faces of rocks, surfaces 
of shells, ete. When disturbed 
or frightened, the animal with- 
draws itself into its tube, which 
it closes by means of a plug or 
operculum, and thus places it- 
self in a position removed from 
its enemies. When extended Genie 
it presents a beautiful appear- 
ance, with its distended crown of brilliantly-tinted 
branchise—red, yellow, purple, and brown—con- 
sisting of some thirty or more delicate feathery fila- 
ments. The brown sea-wrack occasionally brings 
to us, although less frequently than on the New 
England coast, small rounded bodies, not much 
larger than a pin’s head, which to every appearance 
10 
