58 ANIMAL PAEASITES. 



the lateral walls of this impression become converted into the 

 body (central body) of the future cystic worm, and the remain- 

 ing portion of the embryonal vesicle, which is not inverted, or 

 which is not implicated in the individual metamorphosis, becomes 

 the so-called caudal vesicle. That the inversion and the first 

 formation of the head must frequently commence at the anterior 

 surface of the vesicle, is seen distinctly in the Cestodea of Arion 

 empiricorum, for in these the six small hooks are placed in pairs 

 at the point of transition of the body or central body just men- 

 tioned into the caudal vesicle pf the worm, and indeed at a 

 distance of 0'02 millim. from each other. Their apices are 

 often turned away (Leuckart). 1 But for this very reason, as 

 these booklets are situated upon the central body, and this is 

 lost during the conversion of the cystic worm into a tape- 

 worm, they will necessarily be sought in vain on the last segment 

 of the future tape-worm colony, which Meissner supposed would 

 be the case. The central body and the head are always de- 

 veloped in the interior of the embryonal vesicle, and during the 

 whole period of the vesicular state the head always has its apex 

 directed towards the caudal vesicle, even when the central body, 

 which encloses the head in the form of a hollow canal, has not 



1 Although this description and mode of comprehension differ from those adopted by 

 Stein and Meissner, yet I even previously understood the processes in the way just 

 indicated, which has now been confirmed by R. Leuckart's independent observations upon 

 the Cestodea of the lungs of Arion empiricorum var. ru&ra, which I have sought for in 

 vain in my locality. Stein supposes the embryonic booklets to be lost and taken up by 

 the formative materials of the enveloping cyst, but Meissner had already ascertained 

 that they remain. It is true that Meissner misplaces them on the last third or fourth 

 part of the body. But at the same time we must bear in mind that in all Meissner's 

 investigations the true caudal vesicle was usually torn away by pressure, and entirely 

 overlooked by him. According to Leuckart, when the entire worm, freed from its 

 envelopes, is laid in water, it is observed to protrude its head. The head, central body, 

 and caudal vesicle are then preserved; the latter is spherical, pisiform, very thick, rich 

 in fat-corpuscles, and very clearly distinguished from the central body. Lastly, Von 

 Siebold has also understood Stem's Cestodea from Tenebrio molitor in a different way 

 from Stein. He regards the so-called cystic tail, spoken of by Stein, as belonging to the 

 body of the embryo. I might also adopt this notion. If we do this, we must assume 



1. That Stein has erroneously described the enveloping cyst instead of the caudal vesicle. 



2. That the agreement in the histological nature of the enveloping cyst with that of the 

 parenchyma of the body of the Tenebrio, described by Stein, cannot be so remarkable as 

 he supposes ; and 3. That there are also cestoid embryos which produce the taenioid head 

 on their hinder and not on the anterior part. As matters stand, no definite opinion can 

 be formed, and a new investigation of the cestoid worm of Tenebrio molitor would be 

 very desirable. 



