226 



GENERAL ZOOLOGY 



FIG. 110. Flat- 

 worm, x 10 



found among the others. This is most likely to belong to the 

 species Dendrocoe'lum lac'teum (Fig. 110). It is nearly color- 

 less except for a clouded middle region. Upon 

 examination with a simple lens an observer may 

 distinguish the organs of the interior with 

 remarkable clearness. The mouth is in the 

 middle of the under surface. It is at the end 

 of a short proboscis (the pharynx) which may 

 be rolled inside out. Microscopic food entering 

 the mouth may pass forward through a slender 

 tube, and into all of the many branches, or it 

 may pass backward by two tubes and into their 

 branches. Since every part of the body has its 

 branch from the digestive tubes, the animal 

 does not need a blood-system. At the anterior 

 end beneath, there is a shallow sucker for hold- 

 ing on to stones or plants. Above there is a 

 pair of small eyes of very simple structure. The body is not 

 divided into somites. Each individual is hermaphroditic. 



The class to which Dendrocoelum lacteum belongs is called 

 Turbella'ria (Lat. turbo, a whirling, referring to the move- 

 ment of water about the mouth). 



The Trematode. Animals of the appearance of Dis'tomum 

 somater'ice (Fig. Ill) are not often seen, because they are 

 parasitic in the bodies of other animals. This species infests 

 the bodies of mussels, and as already stated, is now believed to 

 be the indirect cause of pearls being found in those animals 

 (see p. 174). The figure represents the parasite in a stage 

 which is nearly adult. 



The ventral surface of the body has two sucking-disks, one 

 at the anterior end (Fig. Ill, 1) and one near the middle (Fig. 

 Ill, 5). They are for holding on. The mouth is at the cen- 

 ter of the anterior disk, and is connected by a short tube with 

 a wide pharynx (Fig. Ill, 2). The gullet is divided into a 



