CRUSTACEANS AS PARASITES ‘ 197 
formed by the modification of parts of two limbs. The piercing 
jaws are enclosed in a sharp beak-like projection. 
A larger amount of modification is fourtd in the female parasite 
(Achtheres) depicted in fig. 1144, and which is not infrequently 
found attached to the gills or living in the throat 
of the perch. Creatures of the kind also infest a 
large number of marine fishes. One (Lernea, fig. 
1145) is sometimes found attached to the eye of 
the sprat, and, as it is phosphorescent, the little 
fishes which harbour these unwelcome guests are 
known to fishermen as “lantern sprats ”. 
BaRNACLES (CIRRIPEDIA) AS PaRrasiTES.—Some 
of the members of this group have undergone an 
extraordinary amount of degeneration as a result 
of parasitism. This is carried to an extreme in 
a form (Sacculina, fig. 1146) that is sometimes gig s144—A Perch: 
found projecting from the under side of the tail of {house ’. (aches 
the Shore-Crab (Carcenus menas). Only a pro- 
fessed zoologist would suspect it to be a Crustacean, for in appear- 
ance it is simply a rounded bag, which dissection shows to be 
provided with numerous branching root-like threads that grow 
through the body of the unfortunate host, extending even to the 
tips of the limbs. A study of its weird life-history (fig. 1146) 
definitely proves that it is really a distant relative of its unfor- 
tunate host. From the egg hatches out a 
little larva, of the kind (nauplius) typical for 
lower Crustaceans (see vol. iii, p. 364). After 
undergoing several moults it assumes a form 
not unlike that of a mussel-shrimp, and con- 
tinues to swim about for three days or more. 
At the end of this period it seeks a very : 
young crab, and fixes itself by means of a a ee 
feeler to the soft membrane at the base of 
one of the bristles on a limb or on the back of its victim. The 
hinder part of its body is then thrown off bodily, and the organs 
contained in the remainder fuse together into a soft mass. Around 
this a membrane is developed, part of which becomes converted 
into a tube that is pushed into the interior of the crab. Through 
this the soft substance of the parasite squeezes itself. Within the 
body of its host it migrates to the region of the intestine, in 
