30 



OVUM. 



not itself converted by any direct process of 

 development or metamorphosis into a perfect 

 Distoma, but lias gradually formed from germ- 

 cells within it a progeny, sometimes of one, 

 more frequently a number of bodies, which, 

 when they arrive at maturity, present each one 

 an external form and internal structure and 

 locomotive powers, entitling them to be con- 

 sidered as independent animals. Nor are these 

 directly converted into Distomata ; but again 

 there is formed within the body of each, and 

 in the same gradual manner from germ-cells, 

 a new progeny of animals nearly similar to 



those producing them and equally differing 

 from the complete Distomata. Each of this 

 new progeny, as it increases in size, has formed 

 within it by development from germ-cells the 

 third progeny of the series, and the last of 

 the cycle ; but these are different from their 

 immediate parents, and in their internal or- 

 ganisation scon manifest the type of the 

 true Trematode. These animals are endowed 

 for a time with very active locomotive powers, 

 to which a long caudal appendage con- 

 tributes ; their two progenitors have been 

 confined in the parasitic condition, but these 



Fig. 28. 



Series of changes in the development and generations oj Distoma. (From Steejistrnp.~) 



o, Ovum with embryo or larva developed in it. e, this embryo in a free moving state ; e', another 

 embryo in its interior. (These are of Monostomum mutabile, from V. Siebold.) 



E, this last embryo farther advanced. 1, first stage, soon after it becomes free ; 2 and 3, farther on, 

 with g, the second generation, within them in various stages. 



o, 1, one of this second generation at an early period of its advancement; 2 and 3, farther on, with 

 c, c, Cercariae or Distoma-larvse, within them ; g', one of the granular globules from which the Distoma 

 larv and previous generations arise near the posterior part of the body. 



c, one of the Cercarise or Distoma larva with its caudal appendage, p, the same, passed into its en- 

 cysted or pupa state, having previously lost its tail. 



D, Distomata. 1, young Distoma immediately after it has quitted the cyst, and has penetrated a 

 short distance into the body of the snail ; 2, Distoma found deep in the viscera. 



are in general freed from confinement, and 

 move about with great vivacity for a time 

 in the water surrounding the animals which 

 their progenitors have infested. In this state 

 they have long been known as Cercariae, and 

 as they have been supposed to be the young 

 of Distomata, have attracted peculiar notice 

 among Helminthologists.* 



The free Cercaria? are not, however, directly 

 converted into Distomata; but appear always 

 to undergo a previous metamorphosis in a 

 chrysalis state, or enclosed in a pupa cyst. 



* Nitsch, Beitrag zur Infusorienkunde, &c., Halle, 

 1817; Bojanus, in Isis, 1818; A remarkable and 

 interesting series of papers by V. Baer, in Nov. Act. 

 Nat. Curios. 1826, vol. xiii. ; Riul. Wagner, in Isis, 

 1834 ; V. Siebold, in Burdach's Physiol. vol. ii. of 

 German edit. p. 187., or vol. iii. of French transl., 

 p. 32., &e. 



Previous to the formation of this cyst the 

 Cercariae adhere to, and bore into, the sub- 

 stance of the animal infested by the Disto- 

 mata; the tail is cast off, an exudation from 

 their own bodies forms the cyst, which en- 

 closes them : within this they remain for 

 many weeks, and even months, moving all the 

 while, and undergoing changes of develop- 

 ment, by which they are at last converted 

 into the complete Distoma. 



The greater number of the observations 

 from which this remarkable process of gene- 

 ration has been ascertained to occur are due 

 to V. Siebold and Steenstrup ; but the whole 

 succession of changes has not yet been ob- 

 served in any one species, and it is to the 

 latter observer especially that the scientific 

 world is indebted for the ingenious com- 

 bination and interpretation of the scattered 



