248 PARASITES. 



The total length of the finest indi%ddual specimens may approach six 

 or seven lines^ the breadth of the shield or upper portion bemg tovv^ards 

 two. The whole is divided into three distinct sections : first, the shield or 

 body, somewhat orbicular, thin and slightly concave, apparently com- 

 posed of several shelly plates, faintly indicated, and with a clear mar- 

 gin ; secondly, a much smaller portion, by some called the abdomen, and 

 next the tubular appendages. On the edge of the front are two anten- 

 nular organs, which, externally, seem .fringed, and behind these are the 

 two narrow oval black eyes, which, even under a moderate magnifier, 

 seem confluent, though actually separate ; thence inducing various obser- 

 vers to class this subject with the Monoculi ; three pair of the limbs be- 

 longing to the under surface issue from the abdomen ; a fourth is per- 

 haps armed with a kind of horny spines ; colour of the whole animal 

 lighter or darker wax -yellow. 



This creature darts quickly through the water, or uj) the side of the 

 glass, considerably above the surface, where, incapable of returning, it 

 adheres and perishes. 



Various fishes, especially the cod, are infested by the Caligm, from 

 which, having no means of liljeration, they gradually pine away, languish 

 and die. 



But, as if in just retribution, the CaJkjus itself is infested by a 

 minute white leech, in vast multitudes, little exceeding a line in length. 

 While occupied with the Caligus in 1824, I observed this parasite, which 

 has been since more distinctly described and named Udonella, by an 

 industrious naturalist, Dr Johnston. I cannot afiirm, however, that the 

 two may be identified as a single species. 



At certain seasons of the year, the female Caligus bears two cyhn- 

 drical tubular appendages, appearing deeply indented throughout by 

 numerous external circles. 



However, these appendages are but temporary, not permanent, 

 though the delicacy of the animal, preventing its survivance more than 

 a few days, precludes observation alike of their origin and of their ob- 

 literation. 



Having isolated a specimen bearing such appendages, in a watch- 



