PHYLUM PLATYHELMINTHES 239 



genera of the family liixtomiila' by being hermaphrodite (two of 

 the genera JJi/Jicti-.i<t and KulULrria- having the sexes distinct), 

 and by having both anterior and posterior suckers, but no retractile 

 tentacles (present in the genus UhopalopJiorus). The Distomidcs 

 are a family of the order bit/met im., and differ from the remaining 

 families by the union of the following characters : The anterior 

 sucker is perforated for the mouth ; the posterior sucker is on the 

 ventral surface and not terminal : the sexual aperture is usually on 

 the ventral surface and in the anterior third of the body, in front of 

 the posterior sucker ; the eggs are almost always furnished with 

 an operculum. 



Tccnia solium is one of the many species of the genus Ttrnia, 

 of the family To n unlit . which is distinguished from the other 

 families of Cestodes by the possession of four suckers, with or 

 without a circlet of hooks, and by the development of well-defined 

 proglottides which become separated off when mature. 



3. GENERAL ORGANISATION. 



General External Features. As the name of the phylum 

 denotes, the body in the Platyhelminthes is, in the great majority 

 of cases, much compressed in the dorso- ventral direction; very 

 thin, so that when very short it may be described as leaf- 

 like, or, when more elongated, as ribbon-like : or thickish in 

 the middle and becoming thinner towards the margin. .Some, 

 h >\vever, have the body comparatively thick, usually with a certain 

 amount of dorso- ventral compression: a few are approximately 

 cylindrical or fusiform. The symmetry is always lilitfcral (p. 41), 

 the radial arrangement of parts so prevalent in the Coelenterata 

 and primarily, as we have seen, associated Avith a fixed or stalked 

 condition, never being observable. A Flat-Worm has dorsal and 

 ventral surfaces, right and left sides or borders, and anterior 

 and posterior ends. The anterior end is that which is directed 

 forwards in ordinary locomotion : it usually has one or other of 

 the characters of a head-end : but a distinct head is rarely dis- 

 tinguishable, and the mouth, when present, is usually placed some 

 distance back on the ventral surface. 



In the Turin'] I aria (Fig. 186) the leaf- form is the prevailing one, 

 a general shape resembling that of Planaria being very common. 

 In many, however, the body is greatly elongated, and it may 

 assume tJie shape of a thin ribbon with puckered edges, as 

 in some marine forms: or may be thickened and band-like, 

 as in the Land Planarians ; or it may approach the shape of 

 a cylinder, as in some Rhabdocosles. A head-region is not 

 usually distinct ; but there is always something to mark off 

 the anterior from the posterior end a difference in shape, the 

 presence of eyes, and, sometimes, of a pair of short tentacles ; 



