286 THE ANIMAL PARASITES OF MAN 



transverse diameter is equal to their dorso-ventral diameter. As a 

 rule the segments, singly or several united together, detach them- 

 selves from the posterior end, in many cases only after complete 

 maturity is attained, and in others much earlier; they then continue 

 to live near their parent colony, to still call it by that name, in the 

 same intestine and continue their development. Even when evacuated 

 from the intestine the proglottids under favourable circumstances 

 can continue to live and creep about, until sooner or later they 

 perish. 



The first proglottis formed, and which in a complete tapeworm 

 [i.e., sexually complete] is the most posterior, is as a rule smaller and 

 of different shape, it also frequently remains sterile, as likewise happens 

 in the next (younger) segments in a few species ; otherwise, however, 

 sooner or later the generative organs develop in all the segments, 

 mostly singly, sometimes in pairs ; in the latter case they may be quite 

 distinct from each other or possess some parts in common. The 

 term " mature " is used for a proglottid that has the sexual organs 

 fully developed, while " gravid" is used for one containing eggs. 

 Most of the species combine male and female genitalia in the same 

 segment, only a few are sexually distinct (Dicecocestus). In the 

 hermaphrodite species one male and one female sexual orifice are 

 always present, and, in addition, there may be a second female orifice, 

 the uterine opening; as a rule, however, this is lacking, and in one 

 sub-family, the Acoleince, to which also the genus Dicecocestus belongs, 

 the other sexual orifice, the opening of the vagina, is also absent. 

 The position of these orifices varies ; the cirrus and vagina usually 

 open into a common atrium on one lateral border or on a surface 

 of the segments ; the orifice of the uterus may be on the same 

 surface or on the opposite one. 



The surface on which the uterus opens is termed the VENTRAL 

 SURFACE; if this orifice is absent, one must depend on the ovary, 

 which almost always approaches one of the two surfaces ; this surface 

 is then called the ventral. 



The length of the Cestodes independently of their age depends 

 on the number and size of the segments, as well as on their 

 contraction ; the smallest species (Davainea proglottina) is 0*5 to 

 ro mm. in length; the largest may attain a length of 10 m., and 

 even more. 



The entire superficial surface of the tapeworms is covered with 

 a fairly resistant and elastic layer, which exhibits several indistinctly 

 limited layers and which is usually called a cuticle, which also covers 

 the suckers, and is reflected inwardly at the sexual orifices. In some 

 species fine hairs appear, either on the entire body or only in the 

 region of the neck, on the external surface. In the cuticle there can be 



