92 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



3. Study of the microscopic structure. The slides contain sections in front 

 of, through, and behind, the pharynx. All three sections should be made use 

 of in the following examination. 



The structure of Planaria differs from that of Hydra in that a third layer, 

 the mesoderm, composed of muscles and connective tissue is present between 

 the ectoderm and entoderm. Planaria is therefore a triploUastic animal, con- 

 sisting of three germ layers. In the sections the ventral side is the flattened side ; 

 the dorsal side the convex side. 



a) Ectoderm: This forms a thin layer of ciliated epithelial cells, the epidermis. 

 In the epidermis may be distinguished rodlike bodies, the rhabdites, which when 

 discharged swell and form mucus and mucus cells, containing granules which are 

 also discharged to form mucus. These mucus cells are unicellular glands. 



b) Mesoderm: Just under the epidermis is a thin layer of circular muscle 

 cells. Within these is a rather indefinite layer of longitudinal muscle cells 

 which appear as circles. There are also dorsoventral muscles, extending between 

 the dorsal and ventral sides, between the branches of the digestive tract. The 

 rest of the mesoderm forms a characteristic connective tissue, called parenchyma, 

 which consists of branching cells and fills up all space between the ectoderm and 

 entoderm so that a coelome is not present. 



c) Entoderm: Each section contains several circular hollow masses which 

 are the cross-sections of the digestive tract. The wall of the digestive tract 

 consists of a single layer of elongated epithelial cells, which are the entoderm 

 cells. These are generally so vacuolated and full of food material as to be almost 

 unrecognizable. 



d) The pharynx: The pharynx lies in a central cavity, the pharyngeal cham- 

 ber. On some slides the ventral opening of this chamber, the mouth opening, 

 may be present. The pharynx is an outgrowth of the body wall which has come 

 to lie in a depression of the body wall, the pharyngeal chamber. The pharyngeal 

 chamber is therefore lined with ectoderm, and the pharynx is covered and lined 

 with ectoderm, the same as the outer layer of the body. The remainder of the 

 pharynx is mesodermal, consisting of circular, longitudinal, and radiating muscle 

 fibers, which the student should by this time be able to recognize, and parenchyma 

 between these. 



e) The ventral nerve cords: These can usually be recognized (best on sections 

 anterior to the pharynx) as pale, round areas, near the ventral surface, about 

 one-third to nearly one-half of the distance out from the median line. The 

 pale portion consists of nerve fibers; often a few nerve cells, large cells with 

 large nuclei, will be found along the dorsal border of the mass of fibers. Make 

 a diagram of the cross-section through the pharynx. 



4. Feeding experiment. Demonstration. Observe the behavior of Planaria 

 when meat is put into the pan. Do they recognize the presence of food? 

 How? After the animals have attached themselves to the meat gently lift one 



