112 A BIOLOGY OF CRUSTACEA 



host. This is usually a worm or a mollusc. The nauplius breaks into 

 the body of the host, then loses its appendages and takes up 

 residence in a blood vessel. Two long processes then develop from 

 the surface of the parasite, and are thought to be concerned with 

 the absorption of food from the blood of the host. The body 

 gradually becomes longer, and an adult develops, which breaks out 

 of the host, leaving the cast larval skin behind together with its 

 long absorptive process. There is no digestive tube in the adult, so 

 that it cannot feed, it can however swim actively and forms the 

 main distributive phase of the life cycle. This is a marked contrast 

 with most parasitic Crustacea, which rely upon their larvae to dis- 

 tribute them while the adults are usually so grossly modified as 

 to be incapable of extensive locomotion. 



The life histories of some species of Lernaeocera are remarkable 

 because two distinct hosts are involved. The adult female of 

 L. branchialis (fig. 49A) lives on cod, whiting and pollack (Gadus 

 spp.). Its body is large, swollen and deformed, with two long coiled 

 egg strings. The head is buried deeply in the tissues of the host and 

 penetrates the main artery leaving the heart, sometimes actually 

 entering the cavity of the heart. The copepod feeds on the host's 

 blood; it takes its meals infrequently, but digestion is complete, and 

 there is no anus for the expulsion of undigested material. 



The eggs produced by the female Lernaeocera give rise to a 

 nauplius stage which lasts for about a day before it moults and 

 changes into a copepodid. The copepodid swims actively and 

 eventually attaches itself to the gills of a flounder (Platichthys 

 flesus). When the copepodid moults it produces an attachment 

 thread from a gland on the front of its head. This thread penetrates 

 the skin of the host and the attached larva is then known as a 

 chalimus. This is a modified copepodid stage which lacks swimming 

 setae on its legs. Four moults are passed in the chalimus stage, then 

 the males become sexually mature and produce sperms which they 

 transfer to the females. Both males and females develop swimming 

 setae on their legs and can swim actively. The males die after 

 mating, but the females swim away in search of a member of the 

 cod family. When a cod is found the female enters its gill chamber 

 and attaches itself near the front of the fourth gill arch. Once 

 attached the female begins to grow. The body becomes longer and 

 bends into the characteristic shape of the mature female. The head 

 penetrates the tissues of the host and antler-like processes grow out 

 to act as anchors. Eventually all traces of appendages are lost and 



