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— [Illustration: $1] [Illustration: $1] (_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—