134 



COLLEGE ZOOLOGY 



are well known, at least by name. Parasitic 

 species are of great interest and very impor- 

 tant economically. Their mode of life has 

 brought about various specializations, such 

 as enormously increased powers of reproduc- 

 tion, and extremely complicated life cycles, 

 involving in certain cases three or four dif- 

 ferent species of hosts and intermediate 

 hosts. The relations of some species to hu- 

 man health and to the rearing of domesti- 

 cated animals constitute a large part of 

 what is known as economic zoology and 

 medical zoology. 



Three classes of Platyhelminthes may be 

 recognized: these are (1) Turbellaria, (2) 

 Trematoda, and (3) Cestoda. Most of the 

 Turbellaria are free-living and inhabit either 

 fresh or salt water; a few live in moist soil, 

 and a few are parasitic. The trematodes and 

 cestodes are all parasitic. 



PLANARIAN-A FRESH- 

 WATER FLATWORM 



The commonest fresh-water planarian in 

 the United States is Dugesia tigrina (Fig. 

 68). It lives on water plants in ponds, and 

 along the shores of ponds, lakes, and rivers, 

 and in small streams under stones. Its up- 

 per surface is brown or mottled and irreg- 

 ularly spotted with white, and its under sur- 

 face is white or grayish. The body is 

 bilaterally symmetrical, broad and blunt at 

 the anterior end, and pointed at the pos- 

 terior end, and may reach a length of from 

 15 to 18 mm. 



The anterior end of the animal is quite 

 distinctly the head. At each side of the 

 head is a sharp projecting auricle. It con- 

 tains a variety of sense cells. A pair of eyes 

 (Fig. 68) is present on the dorsal surface 

 near the anterior end. The mouth is not on 

 the head, but near the middle of the ventral 

 surface. It opens into a cavity which con- 

 tains a muscular tube, the pharynx (Fig. 

 68), attached only at its proximal end. The 

 pharynx consists of a complex of muscle 



layers and many gland cells. By means of 

 the muscles, the phar}-nx can be thrust out 

 of the mouth some distance when feeding. 

 On the ventral side, posterior to the pharynx 

 is a smaller opening, the genital pore; this 

 is present only in sexually mature individ- 

 uals. The ventral surface of the body is 

 covered with cilia, which play some part in 

 locomotion; however, the chief method of 

 locomotion is by almost imperceptible mus- 

 cular contractions. 



Planarians, like other flatworms, possess a 

 mesoderm. The tissues of mesodermal 

 origin, lying between the body wall and the 

 intestine, consist of a fibrous mesh, in which 

 are embedded fixed cells whose processes 

 anastomose, and free cells that can move 

 about in amoeboid manner. This mesoder- 

 mal network of connective tissue is called 

 parenchyma (mesenchyme). The well-de- 

 veloped muscular, nervous, digestive, ex- 

 cretor\% and reproductive systems are con- 

 structed in such a way as to function 

 without the coordination of a circulatory 

 system, respiratory system, coelom, and 

 anus, which are present in many more com- 

 plex animals. The digestive system consists 

 of a mouth, a pharnyx, and an intestine of 

 three main trunks with a large number of 

 small lateral extensions (Fig. 68). 



The food of planaria consists of animals, 

 living or dead. The pharjnx is protruded 

 into the food; and by a sucking action, 

 microscopic particles are detached and 

 drawn into the digestive cavity. Digestion 

 occurs only within cells lining the simple 

 intestine. There is only one opening to the 

 digestive cavity; as in coelenterates, the undi- 

 gested matter is ejected through the mouth. 



Figure 68. Facing page, a planarian. Diagram 

 on upper left shows the digestive and nervous sys- 

 tems. Diagram on upper right shows part of the 

 reproductive system; and at anterior left side, part of 

 the excretory system. Actually these systems exhibit 

 bilateral symmetry, but portions of each are shown 

 to conserve space. Lower, schematic diagram of 

 pharynx. (After L.H. Hyman, The American Biology 

 Teacher, 1956.) 



