STRUCTURE OF PROGLOTTIDES. 89 



In our Taenia the positions of the hooks are so extraordi- 

 narily various, that we scarcely know how to indicate them satis- 

 factorily. This much is certain, before the hook of the cystic worm 

 buries itself in the intestine, as the hook of the young Taenia, 

 it has first to elevate itself perpendicularly about a quarter of a circle 

 from behind and below, anteriorly and upwards, and afterwards 

 to turn itself round about a quarter of a circle in the horizontal 

 plane. In order to retract its hooks as a tape-worm, the animal 

 must also elevate these hooks a fourth of a circle, at the same 

 time turning itself round about a quarter and retracting itself, 

 and on again immersing the retracted hooks, it must push these 

 forwards and, allowing the hooks to turn one fourth round upon 

 themselves, describe a quarter of a circle in the horizontal plane. 

 This at least applies to the hooks situated on the side of the 

 circle. These movements will be best understood with a wax 

 model. 



The sexual organs consist — 1. Of a symmetrical lateral yelk- 

 stock (vitellogene), placed to the right and left; that is, a perpen- 

 dicular canal, filled with yelk-masses, with dilatations and blind 

 sacs, which run outwards and inwards. Between these, and quite 

 posteriorly, lies — 2. The germ-stock (germigene), which is usually 

 double, rarely single, with its clear, cellular egg-germs ; and above 

 this — 3. The uterus, with developed eggs of various forms, and 

 furnished with a germinal vesicle. At the point in the posterior 

 third of the segment where 2 and 3 meet, and where 5 also appears 

 to come, 4. The posterior end of the vagina is to be found, it forms 

 a long narrow passage, which leads, in a curved form, from the 

 point of union just mentioned to the middle of the segment, 

 and thence outwards horizontally beneath the sac of the 

 penis. Between all these parts, and especially above the 

 uterus, lie, 5. The testis (Hodenschlauche Schultze), which Van 

 Beneden called " cellules transparentes," with seminal cells or 

 long seminal filaments. Sometimes we can only detect upou it 

 a pointed prolongation, sometimes small canals (vasa efferentia), 

 collecting in, G, a common seminal duct, which, runs in trans- 

 verse and parallel convolutions, and, with the vagina, opens into 

 7, the sac of the penis, which opens outwardly above the vaginal 

 orifice in 8, the genital pore, sometimes isolated from the 

 female vaginal orifice, sometimes united with it in a sexual 

 cloaca as in the Taeniae with which we have to do. 



This is briefly the general type according to which the sexual 



