326 ESSENTIALS OF ZOOLOGY 



External Anatomy 



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

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

 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 bilateral. 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 proboscis which is used in feeding. Posterior to the 

 mouth is a small, constricted, scarlike aperture, the genital 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 repro- 

 ductive 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 

 buccal cavity which it nearly fills; a three-branched enteron or in- 

 testine, which branches immediately from the anterior end of the 

 pharynx into an anterior trunk ; and two lateral trunks that turn 

 posteriorly, one along each side of the pharynx, and extend nearly to 

 the posterior end. The pharynx is in the form of a cylindrical fold 



