FLATWORMS 



ing cells. The excretory, nervous, and reproductive systems are embedded in 

 this spongy tissue. The turbellarian is thus triploblastic, or three-layered, 

 in contrast to such a coelenterate as the hydra, which is diploblastic. 



Fresh-Water Planarians. Habitat, General Structure, and Actimties. 



The term planarian is sometimes applied to both polyclads and triclads but 

 usually refers to the triclads alone. Triclads are common in fresh water. 

 Like the hydras, the planarians have long been familiar objects of study to 

 zoologists and are well suited to illustrate the structures and functions typical 

 of the phylum to which they belong. Such species as Dugesia tignna, D. 

 dorotocephala, and Cura foremami are widely distributed in North America. 

 ITiey occur in ponds and streams, usually on the shaded sides of submerged 

 objects. They avoid strong light and are probably nocturnal in many of their 

 activities. In nature they feed on the bottom ooze with its microscopic plants 

 and animals, on dead animals, and on living forms such as small moUusks and 

 arthropods; these they capture by enfolding the prey with the margins of the 

 body and then applying the extruded pharynx. When bits of meat are placed 

 in a dish containing planarians not recently fed, the worms begin to move 

 about and soon most of the individuals will be found with the pharynx at- 

 tached to the food (Fig. 11.2). Species of the genera Dendrocoelum and 

 Procotyla have a special sucker-like organ at the anterior end, by means of 

 which they seize active prey and hold it while the pharynx attaches for 

 feeding. 



Normal, gliding locomotion is brought about by the action of the ventral 

 and lateral cilia, beating in a layer of mucus secreted as the animal moves. 

 A more rapid and active locomotion is effected by a variety of muscular 

 contractions of the body. 



Structures and Functions Related to Metabolism. In feeding, the muscu- 

 lar, tube-like pharynx is extruded from the mouth and attached to the food, 

 which may be partially digested by fluid from the pharynx before it is trans- 

 ferred to the digestive cavity (Fig. 11.2). At rest the pharynx lies withdrawn 

 into a sheath-like cavity; the mouth is actually the external opening into the 

 pharynx sheath, and it is through this opening that the pharynx or proboscis 

 is extended in feeding. Food is drawn in through the cavity within the 

 pharynx itself and then passed into the enteron, with its three main branches 

 and lesser subdivisions. Digestion occurs chiefly, if not entirely, within the 

 gastrodermal cells lining the enteron; extracellular digestion is apparently 

 limited to the preliminary softening or liquefaction of foods by the action of 

 fluids from the pharynx. As in coelenterates, the planarian digestive cavity, 

 extensively branched throughout th^ body, assumes in part the functions of a 

 distributing system; no part of the body of the worm is far removed from a 

 branch of the enteron. The products of digestion, as well as oxygen absorbed 

 through the epidermis, are also distributed by way of the lymph-like fluid 

 filling the interstices of the mesenchymal meshwork. 



The excretory system, as it is usually called, consists of minute tubules begin- 

 ning as flame bulbs in all parts of the mesenchymal region and uniting to form 



317 



