MALFORMATIONS OF THE PROGLOTTIDES. 399 



thus producing each time two chains running side by side, which 

 were, however, so unequally developed that each supernumary chain 

 looked like a short lateral branch. Eecalling the 

 familiar fact that lizards, after losing their tails, not 

 unfrequently produce a double tail, it may perhaps be 

 presumed that the doubling is in the above case also 

 the result of an injury in which the chain had been lost 

 up to the proliferating neck. Perhaps it may even 

 happen, as is probably the case in the lizards just 

 mentioned, that the terminal portion gets torn longitudi- 

 nally, or in some other way irregularly injured. 



In Tcenia ccenurus I have observed a remarkable 

 malformation which one may perhaps regard as a situs 

 inversus. The last eight to ten joints of the chain ex- 

 hibited perfectly normal sexual organs, but in inverted 

 position, inasmuch as the organs which should lie pos- 

 teriorly namely, the female reproductive organs 

 were situated anteriorly. The connection of this terminal FIG. 233. 

 chain with the anterior entirely normal portion was jomt^f^oS 

 effected by a short joint, with only testes and two peri- saginata ( x j). 

 pheral knobs lying opposite one another, which, in spite of their 

 resemblance to genital pores, had neither recognisable openings, nor 

 cirrhi, nor vasa deferentia. 



Also it not unfrequently happens that in a series of quite normal 

 joints a segment is interpolated with only male organs. We have 

 already noticed some other deviations in the formation of the sexual 

 organs (p. 278). 



Most zoologists include in the family Tseniadse only a single genus 

 Tcenia. This genus is, however, so rich in species we know almost 

 250 and these exhibit such striking and deep-seated divergences in 

 armature, sexual organs, form of the egg, and mode of development, 

 that we seem fully justified in splitting up the genus into a number 

 of smaller groups. This is not of course the place to enter specially 

 into the systematic arrangement of the Tseniadse, but we are 

 bound to give a rational treatment to the differences between the 

 various tape-worms infesting man, and that with reference to a 

 natural grouping of the genus. 



The Tseniadse, then, we first divide into two groups (see p. 390), which 

 differ especially in the nature of their development. The first group in- 

 cludes the cystic tape- worms, which are distinguished from the others 

 by a great number of peculiarities, and exhibit developmental phases 

 formerly distinguished as Cystici. Not that the bladder-worm stage, as 



