CHAPTER IV. 



CESTOIDEA. 



The Turbellaria. — The animals which constitute this 

 group inhabit fresh and salt water and damp localities on 

 land. The smallest are not larger than some of the Infusoria^ 

 which they approach very closely in appearance, while the 

 largest may attain a length of many feet. Some are broad, 

 flattened, and discoidal, while others are extremely elongated 

 and relatively narrow. None are divided into distinct seg- 

 ments, except the genus A.laurina^ in which there are four ; 

 and the ectoderm, which constitutes the outer surface of the 

 body, is everywhere beset with vibratile cilia. Rod-like 

 bodies, similiar to those met with in some Infusoria and in 

 many Anjielida, are often imbedded in its substance, and in 

 some genera (e. g., 3Ilcrostomum, Tliysanozoon) true thread- 

 cells occur. Stiff setiie project from the ectoderm in some 

 species. 



The aperture of the mouth is sometimes situated at the 

 anterior end of the body, sometimes in the middle, or toward 

 the posterior end, of its ventral face. In many, the oral 

 aperture is surrounded by a flexible muscular lip, which some- 

 times takes on the form of a protrusible proboscis. 



A definite digestive cavity can hardly be said to exist in 

 the lowest Tarhellaria (e. g., Convoluta) in which the endo- 

 dermal cells are not arranged in such a manner as to bound a 

 central alimentary cavity, and the food finds its way through 

 the interstices of an endodermal parench3^ma. In the higher 

 forms, the alimentary cavity, which may be simple or rami- 

 fied, provided with an anal aperture or without one, is lined 

 by the endoderm, between which and the ectoderm is an in- 

 terspace more or less completely occupied by the connective 

 and muscular tissues of the mesoderm. Hence there is no 

 definite perivisceral cavity. 



