THE FREE-LIVING FLATWORMS (TURBELLARIA) 325 



thereby to move freely through the water, and to the unaided 

 eye look much like infusoria. Planarians have a uniform gliding 

 movement but do not swim about unsupported. In addition to 

 the cilia, remarkably long sensory hairs are present in a few forms. 

 The Turbellaria are richly supplied with various kinds of glands. 

 Slime glands occur all over the body and are especially numerous 

 near the anterior and posterior ends. Other glands form the 

 rod-shaped bodies or rhabdites which are either homogeneous 

 and uniformly light-refracting (rhabdoids), or consist of a hyaline 

 outer layer enclosing a fine granular substance (rhammites). The 

 former are extremely variable in shape (spindle-, egg-, rod-, or 

 club-shaped) and originate either in dermal gland cells or in sin- 

 gle-celled glands within the mesenchyma, especially in the anterior 

 end where the tracts through which they pass to the surface may 

 appear as conspicuous lines. The rhammites are found only in 

 the mesenchyma. Still other glands produce the pseudo-rhab- 

 dites which are irregular in shape, granulated in structure, and 

 have a low light-refracting power. A few forms have nematocysts, 

 or stinging cells, similar to those of the coelenterates, in place 

 of rhabdites. Adhesive cells and adhesive papillae are present 

 in many forms, especially at the posterior end of the body. 

 The external openings, mouth, genital pore, and excretory pores, 

 are extremely variable in position. 



In place of the usual body cavity of higher animals, the space 

 between the body and internal organs is filled with a peculiar 

 connective tissue called mesenchyma (parenchyma). In the 

 smaller forms this tissue consists of a few scattered suspensory 

 strands and the space between is filled with fluid. In others 

 there is a network which encloses spaces filled with fluid and richly 

 supplied with cells. The cells may be vacuolated or otherwise 

 modified. The musculature includes bands of circular, longitudi- 

 nal, and diagonal muscles in the body wall. There are also mus- 

 cles which extend through the mesenchyma or connect with the 

 internal organs. The digestive apparatus includes the mouth, 

 pharynx, and intestine, all of which play an important part in 

 classification and furnish a ready means of distinguishing the two 

 great groups of fresh-water Turbellaria. 



