210 PLATYHELMINTHES. 



present, and may be part of the ovary or distinct from it. Such 

 glands are to be regarded as parts of the ovary, the cells of which 

 are without the capacity of becoming ova, but store up yolk matter 

 and are deposited round the ovuin in the cocoon or egg-shell, and 

 consumed by the embryo during its development. Asexual repro- 

 duction is very often found, and may take place at different stages of 

 the development. The life history is in such cases very complicated. 

 So far as is known parthenogenesis is not met with in the group, 

 unless the germ-cells of the sporocysts and rediae of the Trematoda 

 be regarded as parthenogenetic ova. 



Class I. TURBELLARIA.* 



Free-living Platyhelminthes with delicate, soft, and often leaf-shaped 

 bodies, and with a ciliated ectoderm containing rhabdites and sometimes 

 thread-cells, and with muscular protrusible pharynx. 



The Turbellaria include fresh-water, marine, and terrestrial forms. 

 They are distinguished by the possession of a ciliated ectoderm and 

 soft delicate tissues. They usually have an oval, flattened body, and 

 they reach only a small size. It is exceptional to find organs for 

 adhering, viz., small hooks and suckers. The anterior end of the 

 body is especially sensitive, and generally bears eyes and sometimes 

 a pair of tentacl'es, which in a few cases (Euryleptidae and Pseudo- 

 ceridae) contain prolongations of the enteron. In some forms the 

 dorsal surface is covered with papillae, and these also sometimes 

 contain prolongations of the enteron. A pair of ciliated pits, not 

 unlike those of Nemertines, are occasionally present on the front 

 of the body, around which there may be present a ciliated marginal 

 groove. A sucker is found in some Polyclada on the ventral surface. 



The mouth is on the front, middle, or hinder part of the ventral 

 surface. In the American Triclad Phagocata there are eight or nine 

 pairs of mouths and pharynges in addition to the main one, and the 

 same peculiarity has been observed as an exception in the genera 

 Planaria and Polycelis. The generative openings are on the ventral 

 surface behind the mouth. 



The skin consists of a single layer of cells, or of a finely granular 



*L. v. Graff, Monographic der Turbellarien, Leipzig, 1882. L. v. Graff, Die 

 Organisation der Turbellaria Acoela, Leipzig, 1891. P. Hallez, "Catalogue des 

 Turbellarie's du Nord de la France, ".etc., Revue Biologique dn Nord de la France, 

 Ts. ii., iv., and v., 1892, 3. A. Lang, "Die Polycladen," Fauna and Flora des 

 Golfes von Neapel, 1884. F. W. Gamble, "British Marine Turbellaria," 

 Q. J. M. S., 34, 1893, p. 433; and Article on Turbellaria, in the Cambridge 

 Natural History, vol. 2, 1896. 



