190 PLATHELMINTHES 



immature in the younger segments; those following contain the mature 

 organs, and in the terminal segments these have in most species degener- 

 ated, except the uterus, which is swollen with eggs, each of which usually 

 contains an embryo. In the typical human tapeworms these ripe seg- 

 ments are detached and pass out of the body of the host to the outside, 

 where they break open and the young are scattered. In the Bothrioceph- 

 aloidea, however, the eggs are laid in the intestine of the host and pass out 

 with the feces, the tenninal segments often containing no eggs. In some 

 fish tapeworms the segments are detached before they are ripe and remain 

 free in the intestine. The outer surface of the body is an unciliated 

 cuticula, while the interior contains a vesicular parenchyma, in which 

 muscle fibers as well as the other internal organs and usually also cal- 

 careous concretions lie. No digestive system is present, nutrition being 

 carried on by absorption through the outer surface of the body. The 

 excretory system consists typically of two pairs of longitudinal canals, 

 one pair being near each lateral margin, which open to the outside in the 

 terminal segment. The two canals on each side are not of the same size, 

 the ventral being smaller than the dorsal and often disappearing alto- 

 gether. The dorsal and the ventral pair are very often connected by la 

 cross canal in each segment, and fine branches extend throughout the 

 parenchyma containing flame cells. The nervous system consists of a 

 number of longitudinal nerves extending the length of the body connected 

 in each segment by a ring commissure; in the scolex is a pair of large 

 ganglia forming the brain and usually other ganglia, joined with one 

 another by a complex system of commissures. 



Cestodes, with rare exceptions, are hermaphroditic animals, each seg- 

 ment containing both male and female organs; in some species each seg- 

 ment contains two sets of genital organs, one on each side (Fig. 322). 

 The genital pores are situated either in the margin or in the ventral sur- 

 face of the segment. In the lowest cestodes there are three such pores, 

 through which the vas deferens, the vagina, and the uterus, respectively, 

 open to the outside (Fig. 315, B). In most cestodes the vas deferens and 

 vagina pass to a genital atrium which has a single external opening, and 

 no special uterine pore is present (Fig. 327, B). The arrangement of the 

 reproductive organs is complex and varies considerably in the different 

 groups of cestodes. 



Habits and Distribution. — The typical embryo of the tapeworm is a 

 minute spherical animal called the onchosphere or six-hooked embryo (Fig. 

 311), which is provided with three pairs of locomotory hooks. Having 

 been swallowed by its first or intermediate host, either in drinking water 

 or on the food of the latter, it works its way by means of its hooks into 

 the blood vessels and may be carried with the blood to various parts of 



