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 found in the female parasite 

 (Acktheres) 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 (Lerncea, 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 PARASITES. 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 Fig 1I44 ._ A Perch . 

 found projecting: from the under side of the tail of " Louse " (ACM* 



1 J fercarum), enlarged 



the Shore-Crab (Carcinus mcenas). 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 Fi e- 45.-sprat-" Louse- 



J f 6 _ . 7 , (L*r,u,a). enlarged 



feeler to the soft membrane at the base ot 

 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 



