204 NATURAL HISTORY SOCIETY OF DUBLIN. 



acts have been described and figured by Leuckart. The embryo now 

 only wants to find a suitable host, within whose body it may attain its 

 larval or encysted form ; it is fully prepared, as soon as it obtains an en- 

 trance, to maintain its position by the coronet of hooks with which its 

 anterior extremity is armed. There is under the microscope the head 

 of an encysted worm from the peritoneum of a Callithrix monkey, which, 

 though the immature form of a narrow Tapeworm, serves to illus- 

 trate the manner of anchorage of the larva of Bothriocephalus ; for in 

 this phase of their existence the embryos of Taeniae and Bothriocephali 

 are precisely similar, widely different as are the forms of the so-called 

 " heads" of these worms subsequently.* I have also put on the table 

 a carmine injection of the uterus and testicles of a Tapeworm, and a 

 specimen showing the ovary and eggs of a mature segment of the worm 

 from the Polar Bear. The hosts which answer the purpose of the young 

 Bothriocephali found in animals are some of the marine Vertebrata ; for 

 many cestoid worms are known to pass their larval or immature stage 

 in the liver or peritoneum of fish, where they become encysted, and in- 

 crease in size, but do not become sexually ripe from the absence of the 

 conditions necessary to the attainment of this state ; and should their 

 hosts, the fish, perish without having been devoured by an animal of 

 prey, the sexless cestoidea will perish with them without leaving any 

 progeny. 



To Von Siebold we are indebted for much light on this subject. He 

 observed that in certain neighbourhoods the sticklebacks were infested 

 by a kind of tsenoid parasite, which lies free in the cavity of the abdo- 

 men, and often distends the body of its host to an unusual size. As 

 long as the parasite remains in the stickleback its joints and sexual ap- 

 paratus remain undeveloped'; but in the intestine of many of the water 

 fowl, herons, gulls, and divers, which prey on these sticklebacks, a 

 sexually-matured Tapeworm, recognised by the name of Bothrioce- 

 phalus nodosus, had been known to exist. He discovered that this 

 matured worm was the parasite of the stickleback in its further and 

 final stage of development, entered upon as soon as its former host, the 

 fish, had been digested in the bird's stomach, this process having re- 

 leased the larva from its intra-capsular imprisonment. An encysted 

 worm is found in the liver and peritoneum of certain species of salmon, 

 which will not become mature until it has obtained entrance into the 

 intestine of the pike or perch, where it becomes the broad Tapeworm 

 known by the name of Triaenophorus nodulosus. Another example occurs 

 in the case of Ligula simplicissima, infesting the abdominal cavity of 

 various species of carp, whose sexual organs are and remain undeve- 

 loped as long as the worm resides within the fish ; whereas when the 

 latter is eaten by ducks, waders, or other water fowl, the entozoon 

 conveyed into their intestines attains perfect sexual development. 

 Though these facts do not throw much light on the early history 



* Von Siebold on "Worms," Syd. Soc , p. 35. 



