INSECTS AS PARASITES 189 
fortunate star-fish may also present a number of rounded swellings 
in which are lodged parasitic snails of another species (S¢c/aster 
Linckie) that have undergone much further modification, being 
practically endoparasites in which the proboscis has become very 
long, while the rest of the body is much smaller in proportion, 
and the shell has disappeared. Communication with the exterior 
is still kept up, however, by means of a small hole, in the interests 
of breathing and the getting rid of waste products. From this 
case we pass on to degenerate worm-like snails, which are true 
internal parasites, and have lost most of the typical organs of the 
group to which they be- 
long, though the study 
of their life- histories 
renders no doubt pos- on. 
sible as to their classifi- 
catory position. Their 
bodies hang freely into 
the interiors of their 
hosts, one end_ being 
fixed to the inner side Fig. 1134.—Parasitic Cap-Shell (Thyca ectaconcha) attached to the 
Skin of a Star-Fish (Linckia multiforis); diagrammatic section. D.G., 
of the body-wall of the  pigtsive gland; 6, foot-ganglion; S., S., suctorial disc; of. otocyst. 
same. A degenerate 
of the kind (Extocolax Ludwigit) lives within a species of sea- 
cucumber (MMyriotrochus Rinkiz), and one still more strongly 
modified (Extoconcha mirabilis) within another creature of the 
same sort (Syapta digitata). 
The parasitic habit of the larvae of Freshwater Mussels has 
been dealt with elsewhere (see vol. iii, p. 406). 
N 
Pr 
Mis 4 . 
Ss. 
INSECTS (INsEcTa) AS PARASITES 
Innumerable insects have adopted the parasitic habit, either 
for the whole of their lives or for some particular stage of exist- 
ence. It will only be possible to give a limited number of 
examples in illustration of the more interesting kinds of adap- 
tation. 
Bucs (Hemiprera) aS Parasires.—The fact that the insects 
of this order possess piercing and sucking mouth-parts naturally 
suggests that some of them attack other animals, which is indeed 
the case, though the majority would appear to devote themselves 
