PHYLUM PLATYHELMINTHES 165 



temperatures). They react positively to contact (thigmotropism) 

 and water currents (rheotropism). The responses to chemicals are 

 positive in case of food juices and the like; while they are negative 

 to alkalies, acids, strong salts, alcohol, etc. The common species are 

 Plaiiaria niaculata, P. agilis, and P. dorotocephala. 



External Anatomy 



The body is elongated, flat, broadly wedge-shaped at the anterior 

 and tapering to a point at the posterior end. It is triplohlastic 

 since the ectoderm, endoderm, and mesoderm are all differentiated 

 and present in a clear-cut fashion for the first time in our studies so 

 far. The symmetry is distinctly hilateral. In Planaria maculata 

 there is considerable pigment in the skin; while in Dendrocoelum 

 lacteum there is much less. On the dorsal side of the anterior region 

 are two pigment bodies called eyespots which are sensitive to light. 

 At each side of the "head" region is a pointed, sensitive, extension 

 of the epidermis in the form of a lappet or "ear," called an auricle. 

 These are sensitive to touch and chemical stimulations but not to 

 sound. The mouth is located in the midventral portion of the body. 

 The pharynx may be protruded through the mouth in the form of a 

 long, trunklike prohoscis which is used in feeding. Posterior to the 

 mouth is a small, constricted, scarlike aperture, the geniial pore. 

 Externally the epidermal cells are soft and the general surface is 

 nearly covered with patches of cilia which are cytoplasmic extensions 

 of these cells. These cilia along with muscular contractions accom- 

 plish locomotion. The average length of fully developed active P. 

 maculata is about three-fourths of an inch. 



Internal Anatomy 



The ectoderm covers the outer surfaces of the body and composes 

 the nervous system ; the endoderm lines the intestine and its 

 branches; while the mesoderm constitutes the muscular, excretory, 

 and reproductive systems. The undifferentiated mesoderm lying 

 outside the intestine is composed of a meshwork of large cells and 

 is called mesenchyme or parenchyma. Many of the structures of the 

 animal, which have been observed in none of the forms previously 

 studied, have come into existence with the development of mesoderm. 



The digestive system is composed of a mouth in the midventral 

 position; a prehensile pharynx held in the pharyngeal chamber or 



