378 METAMORPHOSIS. 



canal of one host and thence into a fresh host to encyst. It occasionally 

 however happens that this migration does take place, and the life history 

 of Trichina spiralis then becomes almost identical with that of some of the 

 forms of the third type. Trichina is parasitic in man, and in swine, and 

 also in the rat, mouse, cat, fox and other forms which feed upon them. 

 Artificially it can be introduced into various herbivorous forms (rabbit, 

 guinea-pig, horse) and even birds. 



The sexual form inhabits the alimentary canal. The female is vivi- 

 parous, and produces myriads of embryos, which pass into the alimentary 

 canal of their host, through the walls of which they make their way, and 

 travelling along lines of connective tissue pass into the muscles. Here the 

 embryos, which are born in a very imperfect condition, rapidly develop, 

 and eventually assume a quiescent condition in a space inclosed by sarco- 

 lemma. Within the sarcolemma a firm capsule is developed for each larva, 

 which after some months becomes calcified ; and after the atrophy of the 

 sarcolemma a connective tissue layer is formed around it. Within its 

 capsule the larva can live for many years, even ten or more, without 

 undergoing further development, but if at last the infected flesh is eaten by 

 a suitable form, e.g. the infected flesh of the pig by man, the quiescent 

 state of the larva is brought to a close, and sexual maturity is attained in 

 the alimentary tract of the new host. 



Gordioidea. The free larva of Gordius already described usually pene- 

 trates into the larva of Chironomus where it becomes encysted. On the 

 Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus Isevis or 

 Cobitis barbatula), it penetrates into the wall of the intestine of its second 

 host, becomes again encysted and remains quiescent for some time. Event- 

 ually in the spring it leaves its capsule, and enters the intestine, and 

 passes to the exterior with the feces. It then undergoes a gradual meta- 

 morphosis, in the course of which it loses its ringed structure and cephalic 

 armature, grows in length, acquires its ventral cord, and on the develop- 

 ment of the generative organs loses the greater part of its alimentary tract. 



Young examples of Gordius have often been found in various terrestrial 

 carnivorous Insecta, but the meaning of this fact is not yet clear. 



BIBLIOGRAPHY. 



(383) O. Btitschli. " Entwicklungsgeschichte d. Cucullanus elegans." Zeit.f. 

 wiss. Zool., B. xxvi. 1876. 



(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864. 



(385) T. S. Cobbold. Parasites; A Treatise on the Entozoa of Man and 

 Animals. Churchill, 1879. 



(386) O. Caleb. "Organisation et developpement des Oxyurides," &c. Arch- 

 ives de Zool. expcr. et gt'nci:, Vol. vn. 1878. 



(387) R. Leu ck art. Untcrsitckitiigcn ub. Trichina spiralis. 2nd ed. Leipzig, 

 1866. 



(388) R. Leuckart. Die mcmchlichen Parasiten, Bd. n. 1876. 



