DENDROCCELUM LACTEUM 57 



muscular skin bag and is the main organ of locomotion, lies im- 

 mediately beneath the basal membrane. The movement of the 

 epithelial cilia is of use to the animal only when it glides, ventral 

 surface up, on the surface of the water. A well developed layer 

 of circular muscles situated under the basal membrane is followed 

 inwardly by a much weaker layer of oblique muscles. The well 

 developed layer of longitudinal muscles antagonistic to the cir- 

 cular muscles comes next, and finally a weak layer of transverse 

 muscles, adjoining the mesenchyme or parenchyma. Traversing 

 the whole body of the animal from the dorsal to the ventral 

 surface are the dor so-ventral muscles. 



Parenchyma. The space between the dorsal and ventral 

 transverse muscles is occupied by a mesenchyme or parenchyma. 

 In it the cells producing the rhabdites are situated, together with 

 slime glands which open to the outside on the ventral surface of 

 the animal near the edge. The different organs are also im- 

 bedded in the parenchyma. 



Digestive system. The alimentary canal begins with a 

 mouth situated on the ventral side considerably behind the mid- 

 dle of the body. It leads into a spacious mouth cavity, often 

 called the pharyngeal sheath for the reason that it incloses the 

 protrusible pharynx. The pharynx represents a muscular tube 

 covered with epithelium on the inside as well as on the outside 

 and occupying almost the entire mouth cavity. The opening 

 of the free end of the pharynx is known under the name of 

 pharyngeal mouth, and when the pharynx is protruded to the 

 outside through the mouth proper, the pharyngeal mouth is 

 the organ which grasps the animal used for food. Round the 

 pharyngeal mouth the openings of pharyngeal glands are found, 

 which are situated in the parenchyma. The pharynx leads into 

 a blind stomach consisting of three main tubes with numerous 

 side branches. One of the main tubes is median in position and 

 runs forward almost to the anterior end of the animal, the two 

 others run backward, right and left of the mouth cavity to the 

 posterior end. There is no anus. 



