OVUM. 



27 



the true Cysticercus, to a form in which the 

 caudal vesicle has diminished to such an 

 extent as almost to have disappeared, while 

 at the same time the body has been divided 

 into segments by transverse grooves, as in 

 the Taenia ; and in some instances these seg- 

 ments have even acquired sexual organs while 

 the animal was still encysted, a circumstance 

 which has never been observed in any true 

 Cysticercus. 



Fig. 25. 



Cysticercus fasclolaris of tlie Mouse, and Ticnla 

 crassicollis of the Cat. 



A, Cysticercus fasciolaris from the liver of the 

 mouse, natural size. B, the head of the same, 

 magnified. (From Dujardin.) 



c, head and first segments of the body of Tamia 

 crassicollis of the cat, showing the double circle of 

 hooks ; a few of the smaller under circle being seen 

 where one or two of the larger ones have fallen off. 



A close comparison of the structure of the 

 Cysticercus fasciolaris of the rat and mouse 

 in its various stages of development with the 

 Taenia crassicollis of the domestic cat, has 

 shown an almost complete similarity between 

 these animals, and has suggested the view 

 that the encysted Taenia (which the Cysti- 

 cercus fasciolaris in truth is) may attain its 

 full development as a TiEnia in the intestinal 

 canal of those animals which prey upon the 

 smaller Rodentia, in whose liver it begins to 

 be developed in its first simple vesicular form, 

 and gives the greatest probability to the sup- 

 position that there may be a similar general 

 relation between the Cystic and Cestoid En- 

 tozoa, not of the same animals, but between 

 the tapeworms of different tribes of predaceous 

 animals and the vesicular worms of others 

 serving them as food.* 



* Dujardin, Hist. Nat. des Helminthes, 1845. E. 

 lilanchard (who does not appear to have fully appre- 

 ciated the necessity of change of habitation for the 

 entire development *of the trenia), Stir 1'Organisation 

 des Vers, Ann. dcs Scien. Nat. 1848, torn. x. p. o48. 

 V. Siebold, in Zeitsrh. f. Wiss. Zool. 1850, and Ann. 

 des Sciences Nat. 1851. I am indebted to Dr. Henry 

 Nelson, for an account of some interesting researches 

 on this subject which formed a part of his Inaugural 

 J >isscrtation " On the Development of the Entozoa," 

 on obtaining the degree of M. D. at the University 

 of Edinburgh, in 1S6U. The limits of this article 



The different phases of development, there- 

 fore, in which the so-called Cysticercus fascio- 

 laris has been seen in the same and in dif- 

 ferent animals which they inhabit, leave little 

 doubt that they are encysted Taenise, which 

 proceed to a much more advanced stage of 

 development than is usual with the vesicular 

 and encysted form of these Entozoa ; and we 

 are warranted, from the great similarity of 

 structure, in adopting the view that the true 

 vesicular Cysticerci, the Caenuri and Echino- 

 cocci, are morbid or metamorphosed and 

 aberrant conditions of the embryoes of various 

 Tajniae, which may be capable, to a greater or 

 less degree, in different kinds of animals, of 

 multiplying their own incomplete forms by a 

 process of non-sexual gemmation, but which 

 never, in the encysted condition (except in 

 the instances already referred to of the fascio- 

 lated kind), attain to sexual completeness ; 

 but which either undergo a retrograde change, 

 and thus form tumours and various pathological 

 deposits in the seat of their cysts, or become 

 developed to such an extent as to be injurious 

 or destructive to the animal in which they 

 reside.* 



Free Tapeworms. Three principal forms 

 of cestoid worms are now distinguished from 

 one another, viz. Taenise, Bothriocephali, and 

 Tetrarhynchi ; the two first have long been 

 known and sufficiently well characterised in 

 their fully grown condition, though little under- 

 stood in their early or incomplete states ; the 

 history of the third, until recently, has been 

 involved in great obscurity, as it has been 

 most variously described by different ob- 

 servers both in the earlier and more advanced 

 stages of its growth. It appears now to be 

 ascertained that all of these cestoids are com- 

 plete animals, with a single head, a body 

 composed of a multitude of segments, each 

 of which contains male and female sexual 

 organs, which are developed only when the 

 entozoon is living free in the alimentary canal 

 of animals belonging principally to the Verte- 

 brata. The Taeniae inhabit chiefly the alimen- 

 tary canal of mammals and birds ; the Bothrio- 

 cephali and Tetrarhynchi more frequently that 

 of fishes and reptiles, and the latter a few 

 mollusca. The Tetrarhynchi have been more 

 frequently described in the encysted and im- 

 perfect condition than in the full-grown form, 

 and in such varieties, that V. Siebold has 

 mentioned about sixty different kinds of worms 

 described by various authors under distinct ap- 

 pellations, which might, according to him, be 



prevent me from entering into the details of Dr. 

 Nelson's observations, which have not yet been pub- 

 lished. It is enough to mention that a very careful 

 comparison of the Cysticercus fasciolaris of the 

 mouse and rat, in various stages of its development, 

 with the Ta'iiia crassicollis of the cat, enabled him 

 to confirm, in a most satisfactory manner, the view 

 which, unknown to Dr. Nelson, had previously been 

 taken by V. Siebold, that these cystic and cestoid 

 forms are different stages of one and the same 

 animal. See also Leuchnrt on Cysticerci, in Wieg- 

 imnn's and Erichson's Archiv for 1848. 

 * See Gulliver, in Mod. Chir. Trans. 1841. 



