64 MORPHOLOGY OF INVERTEBRATE TYPES 



from the mouth sucker. Of the five openings to the outside 

 found in the majority of trematodes, three are on the ventral 

 surface (the mouth and the male and female genital openings), 

 one at the posterior end of the body (the excretory opening), and 

 one on the dorsal surface (opening of the Laurer's canal). An 

 anus is not present. Locomotion is accomplished by the com- 

 bined action of the muscular system of the body aided by the 

 muscular system of the suckers. 



Integument. Unlike Turbellarians, D. lanceatum has no 

 epithelium, a condition due to its mode of development and true 

 to all Trematodes. The body is covered with a cuticle secreted 

 by special cells which are found between the muscles and in the 

 parenchyma. 



Muscular system. The muscles underlying the cuticle 

 belong to three distinct layers. First come the circular muscles. 

 Then a thin layer of diagonal muscles. The third or innermost 

 layer is composed of longitudinal muscles. Traversing the body 

 of the animal from one surface to the other and attached to the 

 cuticle on both sides are the dorse-ventral muscles. 



The acetabulum or ventral sucker is a shallow cup-shaped 

 muscular organ of attachment composed of radial, equatorial 

 and meridional fibres. Special muscles attached to the acetabu- 

 lum serve for the locomotion of the entire sucker. 



Parenchyma. All the space between the muscular system 

 and the various organs is occupied by a special tissue composed 

 of a homogeneous substance traversed by a network of fibres 

 and containing many nuclei. This tissue is called parenchyma 

 and is a mesenchyme. 



Digestive system. The mouth opens ventrally close to 

 the anterior end of the body and leads into the mouth cavity 

 formed by the mouth sucker. The mouth sucker is composed 

 mainly of radial and a network of diagonal muscular fibres. The 

 anterior edge of the sucker forms an upper lip, the posterior 

 edge a lower lip. Both can be moved separately by means of 

 special muscle fibres in the sucker, belonging to the diagonal 



