SEGMENTED WORMS AS PARASITES 199 
the neighbourhood of the tail, and root-like threads grow out 
from it in all directions, Fed by these it grows rapidly, and 
exerts so much pressure on the muscles and skin which are 
placed between it and the under side of the tail that they be- 
come thinner and thinner. Ultimately, as the final result of this 
process, the parasite projects to the exterior, its roots, however, 
remaining inside the crab. 
Some of the higher Crustaceans belonging to the group of 
Slaters (Isopoda) are also parasitic, and have undergone profound 
modifications. 
SEGMENTED WORMS (ANNELIDA) AS PARASITES 
We are here concerned with various Bristle-Worms (Cheto- 
poda) and Leeches (Discophora). 
BrisTLE- Worms (CuH#topopa) as Parasires.—A number of 
marine worms are external parasites upon hosts of widely different 
nature, including star-fishes, sea-urchins, 
sea - cucumbers, corals, and even other 
annelids. Cases have also been described 
where one species of marine worm lives 
parasitically within the body of another 
species. Much more interesting than 
these, however, are certain small flattened 
creatures (e.g. JZyzostoma, fig. 1147), which 
live upon, or more rarely within, feather- 
stars and sea -lilies, sometimes causing 
gall-like growths that serve as habitations. 
Since these curious little parasites possess Fis. 147—Under Side of Myzostoma, 
enlarged. s, Suckers; /, foot-stumps. 
a small number of foot-stumps, each ter- 
minating in a pair of bristles, they are probably to be regarded 
as bristle-worms that have become modified in consequence of 
their mode of life. And this view is supported by the fact that 
they begin existence as larve which closely resemble those of 
typical worms of the kind. 
One large group of Annelids (Few-bristled Worms, Ozgocheta), 
of which earth-worms and certain freshwater forms are the typical 
representatives, includes a small number of species resembling 
leeches in appearance, as well as in the fact that bristles are 
entirely absent. They are ectoparasites upon crustaceans, the 
