Life-history and Anatomy of the Appendiculate Distomes. 353 



the outside by a uarrow opeuiug at the hinder end of the body 

 (PI. 25, Fig. 1 ap. V, Fig. 2). In the larger worms it was also usually 

 witMrawn, but many individuals were found in which the appendix 

 was extended, and when the free-swimniiug animal was subjected to 

 the pressure of the cover-glass, it would sometimes be rapidly thrust 

 in and out. 



A noticeable characteristic of the worms was their great vitality. 

 Both the free-swimming ones and those contained in Copepods would 

 remain alive in the tow longer than any other animals in it. A jar 

 of tow, the water of which had not been changed, would have but 

 few live animals in it twelve hours after the tow was collected, but 

 twenty-four hours after, many of these worms would still be actively 

 wriggling about, with perhaps only a few small Mollusc-larvae for 

 company. Another characteristic of them was their great activity. 

 Those in the sea-water were constantly moving, although their move- 

 ments consisted only of alternate extensions and contractions of the 

 body which were not accompanied by progression. Those in Copepods 

 were also rarely quiet; they would squirm about in the body-cavity 

 of their host, often ranging from one end of it to the other, and were 

 never observed attached by either sucker to any part of the host's 

 body. Sometimes two worms were seen in the same Copepod and in 

 a very few cases, three were found. Very often the infected Copepod, 

 when it contained a large worm or more than one, would appear ex- 

 tremely sluggish and apparently exhausted, and quite as often it would 

 be dead. In several instances I observed the parasite, by vigorous 

 thrusts, break an opening through the body-wall of the host and pass 

 out into the sea-water. This always took place at a joint between 

 two thoracic segments, either on the dorsal or on the ventral side, 

 and the parasite would emerge sometimes with its forward and some- 

 times with its hinder end first. The elasticity of the chitinous cuti- 

 cula of the Copepod would often hold the escaping worm fast for a 

 while like a trap (PI. 25, Fig. 1), but its powerful struggles would 

 always finally free it and it would become a free-swimming animal. 

 When I first observed this phenomenon I suspected that the pressure 

 of the cover-glass on the Copepod might be the cause of it, but this 

 cannot be the case as I have since found in preserved tow many in- 

 fected Copepods from which the worms were in the act of breaking 

 their way out when they were killed (PI. 25, Fig. 1), 



The fact that the worm occurs in the b o d y - c a v i t y of the 

 Copepod indicates that it is either a Cercaria or a young Distome, 



