H. (I Williamson 51 



they do not occur among North 8ea fish. The infection at Faroe appears 

 to be seasonal, e.g. in summer. 



Fig. 3 exhibits a portion of the flesh of an infected codUng. It was 

 obtained from an Aberdeen fish-yard. The place of origin of the fish 

 was not ascertained. Fig. 4 shows a matted group of nematodes (N) 

 inside the abdominal cavity of the same fish: 

 the worms are attached to the peritoneum {-pt). 



The nematodes do not seem to injure the 

 muscle; they are immature and may be about 

 one inch in length. They resemble in some 

 respects larval stages of Ascaris decipiens 

 described by Linstow. Larval and adult 

 specimens of this worm were found in '*'^'' ^- 



profusion in the alimentary tract of two species of seal. 



The muscle worm is a very resistant form: it survives the preparation 

 of the fish into the smoked fillet. During that process it is in brine pickle 

 for half-an-hour, and afterwards smoked for from three-quarters of an 

 hour to two hours. A fillet prepared in this way was kept for thirteen 

 days: at the end of the period it smelled offensively. Live worms were 

 dissected out at intervals from the first to the thirteenth day. Li a second 

 case the fillet was exposed to the brine and smoke for double the usual 

 time. Thereafter two of four nematodes which were extracted were 

 found to be alive. Nine days later a number of worms were taken out, 

 but all were apparently dead. In a third experiment the cod fillet was 

 dried in the open air: no salt was used. Three months later the fish was 

 very hard. When some of the worms from it were put into sea water 

 they swelled up and became plump. A little movement was detected in 

 one or two cases, but there was no clear evidence that any individual was 

 actually alive. 



The worm-infection of the muscle is not restricted to the codling: 

 it occurs in tusk, saith, and I believe also in haddock. The distribution 

 of the affected members of these species is less clear. 



It is interesting to note that the spotted haddock occurs at Shetland, 

 but is absent from Faroe; the worm-infected codling exhibits the reverse 

 distribution. A locality or environment may thus have a favourable or 

 unfavourable selective influence on its fish population. The two regions 

 form the margins of the 150 miles wide channel through which the 

 warmer and Salter Atlantic current pours into the Norwegian Sea. 

 TrawUng is limited on each side to a zone approximately 30 miles broad : 

 the channel has towards its middle a depth of over 600 fathoms. The 



4—2 



