OCCURRENCE OF THE EMBRYONIC HOOKS IN BLADDER -WORMS. 361 



becomes surrounded by a cellular layer which belongs to the 

 host, and grows out posteriorly into a lappet. The hooks are then 

 cast off, but may be observed for a while in the cellular layer, and 

 the tape-worm head is developed within the embryo. These reports 

 of Stein are unfortunately so far uncertain, that von Siebold thought 

 himself justified in regarding the tail-like posterior appendage as an 

 integral part of the worm. This seems all the more plausible, since 

 it is on it that embryonic hooks are always found. 1 



So much the more grateful is it that we know other Cysticercoids 

 which enable us to decide as to the relation of the six-hooked 

 embryo to the bladder- worm. 



Among these first and foremost is the so-called Cysticercus arionis, 

 which is not unfrequently found in great numbers 2 in the pulmonary 

 cavity of the red slug, and which originates from the Tcenia of a bird, 

 probably a red-shank (Totanus calidris). Meissner found here all 

 the six hooks in their proper arrangement, but he so far misunderstood 

 their position and the structure of the worm, that he overlooked the 

 caudal bladder, and regarded the hooks which occurred at the passage 

 of the latter into the neck as if they were at the posterior end. Such 

 appearances as Meissner saw and figured are not unfrequently seen, 

 but only when the body has been separated by pressure from the 

 caudal bladder, which is all the easier since the cuticular covering of 

 the latter possesses an unusual thickness and firmness, and is besides 

 histologically very different from the former. If, after removing the 

 surrounding connective-tissue cyst, one treat the worm for a short 

 time with lukewarm water, the head can be seen stretched out, and 

 the existence of an actual caudal bladder (Fig. 209, B) is placed 

 beyond doubt. The latter exhibits under its cuticle a cellular layer, 

 whose elements are of considerable size, and enclose a dull granular 

 mass penetrated by fat globules, while the parenchyma of the tape- 

 worm body seems to consist mainly of small clear cells, between 

 which numerous muscle-fibres and calcareous bodies are embedded. 



In its quiescent state, the body of the tape-worm is wholly retracted 

 within the caudal bladder, so that the outer boundary is formed exclu- 

 sively from the thick cuticle above mentioned (Fig. 209, A). The 



1 Moniez has indeed convinced himself ("Essai monogr. Cyst.," loc. cit., p. 78) that 

 the caudal appendage and the cyst of Stein belong to the Cysticercus, and therefore that 

 the latter, essentially like a young Cysticercus pisiformis, lies naked in the body-cavity of 

 the host. 



2 In one case I counted over a hundred Cysticerci side by side on the wall of the 

 pulmonary cavity. See as to this worm especially my work on "Blasenbandw tinner," 

 p. 115, where its structure is for the first time rightly described. The objections which 

 Moniez has lately made (loc. cit., p. 73) to my description apply only to matters of subor- 

 dinate importance, and cannot all be regarded as settled points. 



