154 COMPARATIVE ANATOMY CHAP. 



B. There are 2 longitudinal trunks which open externally at the 

 posterior end of the body by means of a common terminal piece (many 

 Vorticidce, Proboscidea}. 



C. There is a single median principal branch with an opening at 

 the posterior end of the body (Stenostoma among the Microstomidce, 

 Fig. 110, A). 



In the Trematoda also we find typically 2 longitudinal trunks 

 which open externally, either together through a common contractile 

 terminal vesicle of very varying size at the posterior end of the body 

 (digenetic Trematoda, Distoma, Monostoma, Fig. 110, D), or separately, 

 anteriorly and dorsally, by means of 2 widened terminal portions. 



In Distoma hepaticum there is a wider and larger median longitudinal trunk, 

 which stretches pretty far forward, and into which collecting canals enter from all 

 sides. The external aperture lies at the extreme posterior end of the body. 



In the Cestoda (Fig. 110, E) in the simplest cases there are on each 

 side 2 longitudinal trunks running through the whole body, which 

 are united anteriorly in the scolex by a loop. At the extreme 

 posterior end of the body (at the end of the oldest segment) all the 

 four branches open outwardly by means of a contractile vesicle 

 (Tcaniadce, Tetrabotkridw, Tetrarhynchidce). In the Botlmocephalidce, 

 Caryopliyttidcc, and Ligulidce the number of longitudinal trunks is 

 increased to from 10 to 24, which anastomose in a definite way. 

 A contractile terminal vesicle, into which all the longitudinal trunks 

 enter, occurs only in the end of the oldest segment of the tape- 

 worm ; in all other segments, after successive detachments of the older 

 segments, the longitudinal trunks open outward directly and indepen- 

 dently ; some of them, however, may close, and thus end blindly. We 

 have an apparent exception to this rule in Tcenia cucumerina, in which, 

 as segments detach themselves behind, a new contractile terminal 

 vesicle is always formed in the next segment. In many Cestoda, 

 besides the terminal apertures of the water-vascular system, special 

 secondary openings have been observed, generally in great numbers. 

 These are canals, which proceed at right angles from the main canals 

 and open outward through pores. These secondary exits are generally 

 found only at the front end of the body, in the scolex (in Triceno- 

 phorus, many Tcenice, and Tetrarhyncha), less frequently in the proglottides 

 also (Bothriocephalus pundatus and a few other forms). 



The larger canals show a tendency towards the formation of islands in very many 

 Platodes, and especially in the Cestoda. They then break up into a more or less 

 complicated anastomosing network. It is probable that the numerous longitudinal 

 trunks bound together by anastomoses in the above-named Cestoda have proceeded 

 from a few (4) longitudinal trunks by the formation of islands. 



XII. The Sexual Organs. 



All Platodes, with the exception of the genera Microstoma and 

 Stenostoma among the Rhabdoccela and of Distoma hcematobium among 



