SIZE AND FORM OF BLADDER- WORMS. 355 



formed at the base of the protrusion, arise above it on the side walls 

 of the bottle-shaped enlargement, and at first appear as pocket-like 

 invaginations, which afterwards become clad with the above-described 

 muscular wall. He would, in other words, have been convinced that 

 it is the primitive head rudiment which is changed into 

 the tape-worm head at first necessarily invaginated. x 

 Besides, even the adult bladders often show in the position 

 of the head traces of the original state, so that one is in 

 no way warranted in regarding the protrusion as a normal 

 or necessary developmental stage. 



This is true, indeed, only of the majority of cystic 

 worms, but specially of those in which we are now 

 interested. There are, however, exceptions, as for instance 

 the familiar bladder-worm of the mouse (Cysticercus 

 fasciolaris), which becomes Tcenia crassicollis in the in- 

 testine of the cat. In this animal the cylindrical body, 

 which is in other cases hardly ever more than a few milli- 

 metres long, gradually grows to a length of several centi- 

 metres. The receptacle is unable to contain so large a 

 body, and thus the latter comes to be evaginated along 

 with the head at quite an early stage. In this condition FlG - 202. 



. . , . ,., ,. ,, Custwercus 



it persists, and is so like an ordinary tape-worm that its fasciolaris 

 cystic nature escaped detection for a long time. In spite ( nat size )- 

 of this resemblance, the jointed body of this bladder- worm does not 

 pass into the subsequent tape-worm body any more than any other 

 bladder-worm body does. 2 



Mature bladder-worms differ greatly in size and form, as we shall 

 afterwards see. Here we shall only mention that among common forms 

 the smallest is Cysticercus fasciolaris, and C. tenuicollis (the bladder- 

 worm of Tcenia marginatd) the largest. In the former the bladder is 

 usually not so large as a pea, while in the case of the latter it grows 

 occasionally to half a foot in length or more. 



1 My supposition is corroborated by Moniez's own statement ("Essai monographique 

 sur les Cysticerques, " Travaux de VInstitut. zoolog. de Lille, t. iii., p. 41 : Paris, 1880), 

 that he has only imperfectly followed the metamorphosis of the head. He seems to have 

 missed just the decisive stages. None the less did he hesitate at once to condemn my 

 results. According to him, what we have called the head-rudiment is only the cradle of 

 the future head. It is a part of the bladder, arising by invagination, and forming the 

 receptacle ; for what I have described as the receptacle is no special organ, but only an 

 outer muscular layer. I must, however, refrain from entering into a discussion of Moniez's 

 statements, and only remark that I cannot concur with the histological results any more 

 than with the developmental. 



2 In the first German edition of this work I held the contrary opinion, which was 

 based on error, as I have long since pointed out in the case of Cysticercus fasciolaris 

 (Zeitschr. f. wiss. ZooL, Suppl.-Bd. xxx., p. 605, note, 1878). 



