354 H. s. PEATT, 



and that the Copepod is cither its first or its intermediate host. That 

 it is not a Cercaria is shown by the state of development of the re- 

 productive organs. The vesicula seminalis, which is of large size and 

 is a conspicuous object, contained spermatozoa, and the pressure of 

 the cover-glass did, in one instance, cause the cirrus-like genital vesti- 

 bule to protrude beyond the surface of the animal's body and spermato- 

 zoa to be emitted into the water (PL 25, Fig. 2 p). The male 

 genital organs were mature: the female organs, on the other hand, 

 were still immature, the uterus being an extremely delicate tube in 

 which eggs were never seen. 



The worm is, thus, a young Distome whose male sexual organs 

 become mature before the female: the Copepod is its first host, into 

 which it probably entered from the sea-water as a miracidium, and 

 in the body- cavity of which it grew to sexual maturity. To account 

 for the unusual habit it has formed of leaving its host after it has 

 become in part sexually mature and of undertaking a free life, it will 

 be necessary to suppose that the parasite usually becomes so large 

 that it causes the death of the host and then, finding its food-supply 

 cut oiî, it breaks its way out of the host and leaves it. Its food I 

 have ascertained to be the blood in which it is immersed while in 

 the Copepod's body, and which it sucks into its pharynx without at- 

 taching itself to any part of the host's body. While free-swimming 

 it eats nothing. 



What becomes of the worm finally it is impossible to say with 

 certainty, but there can be little doubt that it is swallowed by its 

 final host, which is probably a fish, either while it is leading a free 

 life or while it is still infecting the Copepod, I am very sure the 

 worm does not lead a predatory life while free-swimming, as I have 

 never seen it attached to any animal by either sucker, and its move- 

 ments, although active, are not locomotory in the slightest degree. 

 I am also certain, and for the same reasons, that it does not seek 

 and voluntarily enter its final host. 



A comparison of this worm with the youthful ApoUema appeudi- 

 culatum described and figured by Monticelli (1891, fig. 12) shows 

 it to be, in all probability, the same species. The animal described 

 by that author was found in the body-cavity of a Copepod (Pseudo- 

 calanus elongatus) in the Bay of Kiel by Giesbrecht (1882) in 1877, 

 and identified by Möbius as the youthful Distoma ocreatum. Previous 

 to this time Möbius and Willemoes-Suhm (1871) had frequently 

 taken the same animal in the Bay of Kiel, both in Copepods and free- 



