THE TREMATODA 



1 



FIG. V.Polystomum integerrimum, Frol., from the Cloacal Bladder of the Frog. (After 

 Zeller ; but 5, 6 partially original.) 



1. Dorsal view, showing the alimentary tract, a, mouth, b, pharynx ; between a and 

 b are the four eyes which are very small in the adult ; c, excretory pore of right side ; 

 d, vaginal pores on the " lateral swelling" of the right side ; e, the right limb of the intestine ; 

 /, the transverse anastomoses ; g, the prolongation of the median gut into the caudal disc ; h, 

 caudal disc or cotylophore. 



2. Ventral view, showing the reproductive system, b, penis ; e, common genital pore ; e, 

 sperm duct ; /, vaginal canal, dilated to form a spermatheca ; g, germarium ; h, the genito- 

 intestinal canal, passing from the oviduct to the left limb of the intestine, a small piece of 

 which is shown ; i, testis ; j, vitellarium, extending into the caudal disc ; fc, vitello-duct, the 

 index line passes to the point where the fore and aft longitudinal ducts unite to form the trans- 

 verse duct, from the middle of which a short nearly median duct passes to the oviduct ; I, 

 ootype, containing an egg cell, surrounded by vitelline cells ; the shell glands around the 

 ootype are not shown ; m, uterus, short and slightly undulating, containing a small number of 

 eggs ; n, the germ duct. On the caudal disc are the three suckers (1, 2, 3) on each side ; each 

 surrounding a small hooklet (cf. Figs. 6, 7). Six small booklets are visible between the 

 anterior pair of suckers, and four others, as well as two large hooks, lie between the posterior 

 pair of suckers. 



3. Two individuals copulating. The genital pore of each is placed in contact with the 

 right "lateral swelling "of the other; the left lateral swelling of each is indicated by a and 

 a' . By means of the suckers b, b' , each individual is attached to the wall of the cloacal 

 bladder, a portion of which is represented at c, and being transparent, allows the six suckers 

 of each animal to be seen through. 



4. An egg; a, the germ cell ; b, vitelline cells ; c, egg shell ; d, the "stalk." 



5. A larva, seen from the dorsal surface ; the eyes, ciliated bands, and apical tuft of 

 cilia are shown. 



6. A larva, from the ventral surface. The caudal disc at this stage is without suckers, but 

 is armed with sixteen booklets. 



7. A young Polystomum, in which the two hindmost suckers (2, 3) on each side have made 

 their appearance. The first to appear is No. 3 ; each is developed round a hooklet ; they are 

 here seen in optical section, as the young worm was much stretched ; e, the pair of great hooks 

 have made their appearance in the middle of the posterior region of the disc. The most 

 anterior sucker on each side will develop around the hooklet lying just behind that labelled* 

 d. The pharyngeal sac is everted (a) during the movement of the worm, and in its endeavours- 

 to find a resting-place ; b, pharynx ; c, sac-like intestine, beginning to exhibit central inter- 

 ruptions, which in the course of development are destined to increase in size, while the 

 transverse anastomoses will proportionately diminish. The circular marks represent large 

 cells which, during the course of metamorphosis, accumulate brown granules and concretions, 

 and then drop off into the gut. 



