Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/46

36 The reproduction of the Tape-worm, long involved in obscurity, is now known to be as follows. There exists in the Pig at certain times a sac-like worm called the Cysticercus; this never progresses; but should the part of the pig containing this worm be eaten by man, this sac will be transformed into the Taenia, or Tape-worm. The Leech and the Peripatus are so nearly allied to the Trematoda that they may be regarded as offshoots of that stem. Turning back now to the Turbellarian Worms with a straight intestine (Rhabdocoela), while noticing that the family represented by the Nemertes is given off here (see Tree III.), we see, in following the stem upwards, its importance, in that it furnishes the origin of the Articulated, or Segmented Worms, with their progeny, the Echinodermata and Articulata. Before leaving the Soft Worms, attention must be called to the system of vessels which is found in most, if not all, of this group. It is well developed in the Aspidogaster Conchiola (Fig. 32), a Trematode worm found in the heart-sac of the fresh-water mussel. The worm is shaped somewhat like a vase. Coursing through its body is seen a system of vessels, beginning as large tubes, which, getting smaller, are finally lost as twigs. This system of vessels is supposed to be the same as that observed in an undeveloped condition in the Paramoecium among the Infusoria, and is found also in the Rotatoria, one of the divisions of the Articulated Worms.



The Articulated Worms include the three groups of the Gephyrea, Annelida, and Rotatoria, They are called articulated or segmented, from the fact of their bodies being composed of segments or pieces joined together. This arrangement is carried to the furthest extent in the Annelida, the Nereids (Fig. 34) numbering as many as hundreds in their segments. This segmentation is only just perceptible in the Sipunculus (Fig. 33), one of the Gephyrea, The bodies of the Rotatoria (Fig. 35) are inclosed in a