SPONGES, FLAT WORMS, AND ROUND WORMS 155 



without the coordination of a circulatory system, respiratory 

 system, ccelom, and anus. 



DIGESTIVE SYSTEM. The digestive system (Fig. 77) consists 

 of a mouth (m.), a pharynx (ph.) lying in a muscular sheath, and 

 an intestine of three main trunks (i } i 2 , is) and a large number of 

 small lateral extensions. The muscular pharynx can be ex- 

 tended as a proboscis (Fig. 76, j) ; this facilitates the capture of 

 food. Digestion is both intercellular and intracellular, i.e. part 

 of the food is digested in the intestinal trunks by secretions from 

 cells in their walls; whereas other food particles are engulfed 

 by pseudopodia thrust out by cells lining the intestine, and are 

 digested inside of the cells in vacuoles. The digested food is ab- 

 sorbed by the walls of the intestinal trunks, and, since branches 

 from these penetrate all parts of the body, no circulatory system 

 is necessary to carry nutriment from one place to another. As 

 in Hydra, no anus is present, the faces being ejected through the 

 mouth. 



EXCRETORY SYSTEM. The excretory system comprises a pair 

 of longitudinal, much-coiled tubes, one on each side of the body; 

 these are connected near the anterior end by a transverse tube, 

 and open to the exterior by two small pores on the dorsal surface. 

 The longitudinal and transverse trunks give off numerous finer 

 tubes which ramify through all parts of the body, usually ending 

 in a flame cell. The flame cell (Fig. 78) is large and hollow, 

 with a bunch of flickering cilia (c) extending into the central cav- 

 ity (e). Since it communicates only with the excretory tubules 

 it is considered excretory in function, though it may also carry 

 on respiratory activities. 



MUSCULAR SYSTEM. The power of changing the shape of its 

 body, which may be observed when Planaria moves from place 

 to place, lies principally in three sets of muscles, a circular layer 

 just beneath the ectoderm, external and internal layers of longi- 

 tudinal muscle fibers, and a set of oblique fibers lying in the meso- 

 derm. 



NERVOUS SYSTEM. Planaria possesses a well-developed nerv- 



