Life-history and Anatomy of the Appendieulate Distomes. 379 



testinal tract (usually the oesophagus or stomach) of the following 

 marine fishes ; Clupea Jiarengus, C. sprattus, C. pilchardus, C. nilotica, 

 C. aurita, Älosa vulgaris^ A. ftnta^ Gasterosteus aculeatus, Gadiis 

 melanastomum, Rhomhus laevis and Pleuronectes limandoides, and also 

 of others ; of these fishes, however, by far the most usual hosts are 

 the species of the closely allied genera, Clupea and Alosa. The food 

 of these last named fishes consists of Copepods and other minute, 

 pelagic animals; the first, or possibly the intermediate, hosts of the 

 worms are, consequently, these animals. No cercaria or other meta- 

 morphic form of appendieulate Distomes has so far as I know, been 

 taken, but the young adult with sexual organs more or less immature, 

 has been found in Copepods, Annelid larvae, in Sagitta and Beroe, 

 and also free-swimming. 



The worms have but two hosts, in my opinion, the Copepod or 

 other small, pelagic animal, and the fish. The eggs pass out of the 

 fish into the sea-water and here the miracidium hatches: it, then, is 

 either swallowed by the Copepod, which thus infects itself, or actively 

 seeks its first host, entering its intestinal tract through the mouth or 

 anus. From the intestine it must bore its way into the body-cavity: 

 here, in my opinion, it passes its entire metamorphosis and finally 

 reaches the stage described in this paper. It may finally here become 

 so large that it causes the death of its host, in which case it breaks 

 its way out and passes again into the sea-water. The final host 

 becomes infected by swallowing the young free-swimming worm or an 

 infected Copepod. While free-swimming, the worm does not attack 

 or attach itself to any other animal and does not swim from place 

 to place. 



My grounds for believing that the entire metamorphosis of the 

 worm is passed within the body-cavity of the Copepod are the foll- 

 owing: the parasite must necessarily enter the Copepod either as a 

 miracidium or a cercaria; I hardly think that the cercaria of a worm 

 relatively so large (it may be a quarter or a third the size of its 

 host) could enter the body-cavity of so small an animal as a Cope- 

 pod without rupturing the wall of the intestine or the body-wall, and 

 I have determined by means of sections that both the body- wall and 

 the wall of the intestine of the infected Copepod are always intact; 

 I conclude, therefore, that the Copepod must have been entered by 

 the miracidium. 



The only records of observations on free-swimming, appendieulate 

 Distomes occurring in the literature, so far as I know, are in the 



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