Fig. 153.—Free cercaria of the common liver fluke, showing two suckers, intestine, large glands, and tail. (After Leuckart, 1889, p. 279, Fig. 137.)
Swimming about in the water, the miracidium seeks out certain snails (Limnæa truncatula, L. oahuensis, L. rubella), which it immediately attacks (Fig. 148). The miracidium elongates its papilla and fastens itself to the feelers, head, foot, or other exterior soft portion of the body of the snail; some of the parasites enter the pallial (lung) cavity and attach themselves there. After becoming securely fastened to the snail the miracidium discards its ciliated covering, and shortens to about half its former length (0·07 mm. to 0·08 mm.). The parasites now bore their way into the body of the snail, and come to rest in the liver or near the roof of the pallial cavity, etc.; the movements gradually cease, and we have before us the stage known as the—
(d) Sporocyst (Figs. 149 and 150).—The eye-spots, ganglionic swellings, and vestigial intestine become more and more indistinct, and are finally lost. The sporocyst grows slowly at first, then more rapidly, and at the end of fourteen days or so measures 0·5 mm. The germ cells mentioned as existing in the posterior portion of the miracidium now develop into individuals of a third generation, known as—
Fig. 154.—Portion of a grass stalk with three encapsuled cercariæ of the common liver fluke (Fasciola hepatica). × 10. (After Thomas, 1883, p. 291, Fig. 13.)
Fig. 155.—Isolated encysted cercaria of the common liver fluke. × 150. (After Leuckart, 1889, p. 286, Fig. 142.)
(e) Rediæ (Figs. 151 and 152).—The rediæ escape from the sporocyst when the latter are from two weeks (in summer) to four weeks (in late fall) old. Upon leaving the body of the sporocyst they wander to the liver of the snail, where they grow to about 2 mm. long by 0·25 mm. broad. Each redia consists of a cephalic portion, which is extremely motile, and which is separated from the rest of the young worm by a ridge; under the latter is situated an opening, through which the next generation (cercariæ) escape. The posterior portion of the worm is provided, at about the border of the third and the last fourths of the body, with two projections. There is a mouth with pharynx situated at the anterior extremity, the pharynx leading to a simple blind intestinal sac. The redia, as well as the sporocyst, may be looked upon as a female organism, and in its body cavity are found a number of germ cells, which develop into individuals of the next generation, known as—
(f) Cercariæ (Figs. 153—155).—These organisms are similar to the adult parasites into which they later develop. The body is flat, more or less oval, and provided with a tail inserted at the posterior extremity. The oral sucker and acetabulum are present as in the adult, but the intestinal tract is very simple; on the sides of the body are seen two large glands, but the complicated genital organs of the adult are not visible. The cercaria leaves the redia through the birth opening, remains in the snail for a longer or shorter time, or passes out of the body of the snail and swims about in the water. After a time it attaches itself to a blade of grass (Fig. 154) or some other object, and forms a cyst around itself with material from the large glands, at the same time losing its tail. It now remains quiet until swallowed by some animal. Then, upon arriving in the stomach—of a steer, for instance—the cyst is destroyed, and the young parasite wanders through the gall-ducts or, as some believe, through the portal veins to the liver, where it develops into the adult hermaphrodite.