The royal natural history . cobdella) not unfrequently found living parasiticallyunder the gills of various marine molluscs. In these, thebody is short and broad and capable of but little changein shape at its hinder end; it is furnished witli a suckingapparatus, by means of which the animal adheres to itshost. As stated above, almost all nemertines are of separatesexes; and in some marine species the development of theyoung is so remarkable that it is impossible to pass it bywithout notice. The young which issue from the egg areso unlike the parent, that no one would at first sight supposethe
The royal natural history . cobdella) not unfrequently found living parasiticallyunder the gills of various marine molluscs. In these, thebody is short and broad and capable of but little changein shape at its hinder end; it is furnished witli a suckingapparatus, by means of which the animal adheres to itshost. As stated above, almost all nemertines are of separatesexes; and in some marine species the development of theyoung is so remarkable that it is impossible to pass it bywithout notice. The young which issue from the egg areso unlike the parent, that no one would at first sight supposethem to belong to this group. The larva, as shown onp. 461, bears some resemblance to a helmet, and has beennamed Pilidium. This creature, which is covered withcilia, swims near the shore for some time, while the youngnemertine is developed inside. As soon as this has acquiredfour-eyed nemertine, itg ciH and attaim>(1 a certain stage of maturity, it breaks (enlarged). from the pilidium and starts an independent life. :u,; I. FLAT-WORMS. 459 The Flat-Worms,—Class Platyhelminthes. The flat-worms are characterised by the absence of a distinct vascular system,and by the alimentary canal being either absent or with no posterior outlet. Anervous system is developed, consisting either of a network of nerves, or some-times of a distinct brain and lateral chords. The excretory organs are composedof fine tubules opening to the exterior, and the body-cavity is reduced to a set ofslits in the tissues. For the most part the sexes are united in one a sexual reproduction occurs, accompanied by an alteration of genera-tions. There are three orders of flat-worms, namely, the tape - worms, thetrematodes, and the turbellarians.
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