36 EVOLUIIONTON EIE 
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 Tenia, or Tape-worm.. 
The Leech and the Peripatus are so nearly allied to the Tre- 
matoda that they may be regarded as offshoots of that stem. 
Turning back now to the Turbellarian Worms with a straight 
intestine (Rhabdoccela), while noticing that the family re- 
presented by the Nemertes is given off here (see Tree IL), 
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 Articu- 
lata. 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 Aspido- 
gaster 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 unde- 
veloped 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 articu- 
lated 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 An- 
nelida, the Nereids (Fig. 34) numbering as many a$ hundreds 
in their segments. This segmentation is only just per- 
ceptible in the Sipunculus (Fig. 33), one of the Gephyrea. 
The bodies of the Rotatoria (Fig. 35) are inclosed in a 
