168 COMPARATIVE ANATOMY CHAP. 



body, which till now has been tolerably round, begins to flatten ; the surface 

 in which the mouth lies can as the ventral surface be distinguished from the dorsal 

 surface under which lie the eyes and brain. The mesoderm cells everywhere 

 spread out between the intestine and the body epithelium, and form a continuous 

 mass, which is thicker at the ventral side. Those layers of mesoderm which lie 

 close under the epithelium yield the dermal musculature ; the deeper mesoderm 

 cells yield the body parenchyma, and most probably also the germ -preparing organs 

 of the genital apparatus. 



In a series of Polydada whose embryos leave the egg shell very early as free- 

 swimming Miiller's larvae (Fig. 118), a ring of strong and long cilia which encircles 

 the body arises directly in front of the mouth; this is the so-called preoral 

 ciliated ring, running out round 4 or 8 processes of the body, one of which lies 

 immediately before the mouth and one in the middle line of the back, while the 



FIG. 118. Miiller's Polyclad larva (of Thysanozoon or Yungia). A, Median longitudinal 

 section, g, Brain ; hd, main intestine ; en, endoderm ; ec, ectoderm ; sn, sucker ; ph, pharynx ; pt, 

 pharyngeal pouch ; o, mouth. B, The same from the side. The black line indicates the course of 

 the preoral ciliated ring. 



other 2 or 6 lie laterally in pairs. These processes, with their strong cilia, are 

 drawn in and reabsorbed when the free-swimming larvae sink to the bottom and begin 

 the creeping manner of life. 



The differentiation of the originally single enteron into main intestine and gastro- 

 canals follows in consequence of the growth of mesodermal septa from the periphery 

 more or less far inwards. 



The position of the mouth and pharynx on the ventral surface in the adult 

 animal is determined by the relative growths of the anterior and posterior halves. If 

 they grow equally, these organs lie centrally ; if the anterior half grows the more 

 strongly, they lie posteriorly ; if the posterior grows the more, then they lie anteriorly. 



XV. The Life-history of the Trematoda. 



Whereas from the fertilised eggs of the ectoparasitic or monogenetic 

 Trematoda other Trematoda develop direct without their young being 

 assigned to another animal or host than that occupied by the adult, 

 the development and life -history of the endoparasitic or digenetic 

 Trematoda is remarkably complicated. We choose as an example the 

 tolerably complicated life-history of the fluke, Distoma hepaticum 

 (Fig. 119), which is parasitic in the liver of the sheep, causing the 

 " sheep rot." The eggs of the fluke leave the liver of the host by the 

 bile ducts, pass into the intestine, and are ejected with the excrement. 

 They only develop when they meet with water. In this case there 

 develops in the egg shell (A) a ciliated embryo, which leaves the 



