44 



with the chitinous filament, and leads a pelagic life (Plate 

 IV., fig. 4. Nat. size 2*-') mm.). This condition is fre- 

 quently found in collections of plankton, and unless care 

 be taken ma}^ readily be confused with immature stages 

 of allied forms. I. C. Thompson, r.L.8.,t was the first to 

 recognise certain copepods taken in collections of plank- 

 ton from Liverpool Bay, &.C., as the young of Lerncea, from 

 Claus' figures. The presence of the males of Lerncea in 

 plankton is to some extent accidental, as only the females 

 lead a pelagic life. The males remain on the gills after 

 the females have gone. The result of the examination 

 of the contents of a fine filter, through which the waste 

 water was passed from the tanks containing flounders in 

 the Piel Hatchery, showed that females were always more 

 numerous than males. The ratio, after a number of 

 trials, was found to be one male to twenty-five females. 



At the conclusion of the pelagic life the young Lerncea 

 again fixes itself to the gills of a fish, and the retrogres- 

 sive metamorphosis commences. The parasite buries its 

 cephalo-thorax into the tissues. This region then 

 develops into horns, which are situated one at each side 

 and one dorsal. These pass out at right angles to the 

 body into the tissues of the host. At first they are simple, 

 but by gradual division in each horn they acquire the 

 characters found in the adult (Plate Y., fig. 8). The 

 anterior part of the segment curves over, taking up the 

 position shown on Plate Y., fig. 4. The eyes, antennules, 

 antenna?, mandibles and maxillfe disappear, leaving only 

 the first maxillipedes, which are represented by small 

 hooks in the adult. The free thoracic segments fuse, but 

 the feet remain as in the cyclops stage. The genital 

 segment elongates until fully fifteen times the original 



t Eevised Eeport on L.M.B.C. Copepoda. Trans. L'pool Biol. Soc, 

 vol. vii., p. 212. 



