NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 2.y 



active, doubling and straightening upon itself with the result that it 

 ruptures both its enclosing sheath and the integument of the host. 

 Then it escapes, leaving behind in its late host its spiny cuticle and 

 its nutritive arms, which will no longer be needed. The monstrillid 

 thus, according to Malaquin, makes during its life only two moults, 

 one on entering the host, the other on leaving it. With its liberation 

 the adult becomes at once an active free-swimming copepod (J). It 

 now has only one pair of antennae and four pairs of swimming legs, 

 and it lacks a complete alimentary canal. The body of the female, 

 however, is mostly filled with a great mass of eggs (J, Ov^ ; the busi- 

 ness of the adult is the procreation of more parasites. 



Members of the family Caligidae, mostly parasitic on fish, are also 

 free in the adult stage, but, though the adults are at liberty to leave 

 the host and are equipped with swimming legs, they still depend for 

 their food on the host that nourished them as larvae or on some other 

 fish of the same kind. They, therefore, live largely as free external 

 parasites. The structure and habits of many species of Caligidae have 

 been described by Wilson (1921a), and a detailed account of the 

 larval stages of Caligus ciirtus (O. F. Miiller) is given by Heegaard 



(1947)- 



In Caligus curtus, according to Heegaard, there are two naupliar 

 instars, the second of which goes over directly into a first copepodid 

 without an intervening metanaupliar stage. The first copepodid is 

 followed by a second copepodid, and then come five larval stages in 

 a form known as a chal'muis before the individual becomes adult. The 

 actively swimming first copepodid has the responsibility of finding 

 a host, which will be a codfish. It grasps a scale or a fin ray of the 

 fish by means of its clawed second antennae, and holds on with the 

 maxillipeds. After attachment the copepodid moults into the second 

 copepodid (fig. 10 A). In this stage a gland in the head produces a 

 secretion which will be discharged from the frontal region as a fila- 

 ment (B), which becomes firmly fixed to a scale or a fin ray of the 

 host. The parasite now becomes quiescent and takes no food as it 

 hangs motionless on its attachment line, while within its cuticle a 

 development takes place that will transform the copepodid into the 

 first chalimus. This quiescent period of the copepod (B) is termed 

 by Heegaard and some other writers a "pupa," but, though motionless 

 and nonf ceding, it is not comparable to the pupa of an insect. The 

 insect pupa is a stage in itself during which the metamorphosed larva 

 reverts to the parental form. Each larval instar of any arthropod 

 begins its development within the loosened cuticle of the preceding 

 instar. The copepod "pupa," therefore, is merely the second copepodid 



