653 



THE ANIMAL KINGDOM. 



It is probably derived from the surface of the germ, as in the case of the 

 embryo (ante, p. 649). 



The Cercaria has a body shaped like that of a sexual Trematode, 

 though its proportions may undergo change in subsequent growth. It pos- 

 sesses the rudiments of suckers, digestive tract, and the excretory system, 

 and is furnished with a tail, or in Bucephalus with two tails, attached to 

 the body posteriorly, by means of which it swims when it is set free from 

 its parent and its parent's host. The tail is sometimes of great length, and 

 may be bifid at its extremity. It is in some marine forms armed with 

 spines set in rings or tufts. Occasionally it is nearly obsolete, or even quite 

 so, as in Leucochloridium, and then the Cercaria is probably always 

 quiescent and only passes into its final host when the latter happens to de- 

 vour its parent, as e.g. in the case named. The Cercaria is sometimes 

 armed (except it is said when derived from Rediae] with a boring spine, 

 close to which glands open on either side. The glands have been supposed 

 to secrete either a poison or a cystogenous substance. The free Cercaria 

 becomes after a time quiescent, and either encysts itself in the same host that 

 contains its parent, in another intermediate host, usually non-Vertebrate, 

 sometimes Vertebrate, e. g. the tadpole of frogs \ or on some foreign object, 

 e. g. on stems of grass, &c., in the case of Fasciola hepatica 2 . Encysted Cer- 

 cariae have been found in very various aquatic non-Vertebrates. The cyst 

 is formed by the Cercaria itself, and its material is derived from the 

 secretion of gland-cells. In some forms, e. g. F. hepatica, the gland-cells in 

 question contain rods. These rods may be found afterwards in the walls 

 of the cyst. The tail is cast off during the process of encystment 3 . The 

 Cercaria now grows in size and becomes a sexually immature fluke. A 

 period of rest for this purpose seems necessary, and then as soon as the 

 encysted fluke is swallowed by its final host, a Vertebrate, it attains a 

 sexual condition in the intestine of the latter. It is very rare for it to do 

 so in the encysted condition and to produce ova, as do D. cirrigerum from 

 Astacus and'Z^. agamos from Gammarus pulex. It has been stated that the 

 same Cercaria may give origin to different sexual forms in different hosts 



1 The non-sexual forms ( = Diplostomum ; Tetracotyle] of Holostomum are frequently met with in 

 various Vertebrata. 



8 D. noditlosum is found in the sexual form in the intestines of various fresh-water fishes, e. g. 

 of the Pope, Aerina cernua. Its Cercaria sometimes occurs encysted on the outer surface of the 

 intestine of this fish. It probably enters it at an immature stage in this case, i. e. before under- 

 going encystation in the Paludina, which is the first host. See von Linstow, A. N. H. (4), xii. 

 1873- 



3 It has been stated by several authorities, as by Pagenstecher and Ercolani, that the tail of a 

 Cercaria may in some instances become a Sporocyst and produce germs. The structure of the tail 

 does not favour this supposition. According to Schwarze, it consists of a ' contractile substance 

 occupying the axis and periphery with large vesicular cells between.' Compare Ziegler on the tail of 

 Bucephahts, Z. W. Z. xxxix. 1883, pp. 558-562. For instances to the point, see the work of 

 Ercolani, Dell' adattemento, &c. cited below. 



