76 SPECIES OF CYSTICA AND CESTOIDEA. 



3. The scolices of the Echinococcus veterinorum developed 

 themselves into very small tape-worms, of from one, to one and a 

 half, lines long, which have been shown to be a distinct species, 

 and named Taenia echinococcus. . 



It may seem questionable to many helminthologists and 

 zoologists, that four different kinds of cystic worms, which have 

 hitherto been looked upon as so many distinct species, should 

 only produce one and the same species of Taenits. But I 

 would ask, are the cystic worms which have been termed Cysti- 

 cercus pistformis, tenidcollis, celluloses, and Ccenurus cerebralis, 

 really distinct kinds ? After the present inquiries, this question 

 must be negatived. All these cystic worms are only the 

 degenerated embryos and scolices of a single species of Tania. 

 If they who have always regarded these cystic worms as belong- 

 ing to different species will make the experiment for themselves 

 of detaching the several heads of the four above-named cysticerci, 

 and mixing them together, they will find it utterly impossible to 

 make out any specific difference between them. Further than 

 this, I not only question if the T&nia serrata from the intes- 

 tine of the dog, and the Taenia solium from the human intes- 

 tine are distinct and sharply defined species, (see p. 68), but I 

 also doubt the specific distinctness of the T&nia marginal a from 

 the intestine of the wolf, of the T&nia crassiceps from that of the 

 fox, and of the Ttenia intermedia from the intestine of martens 

 and polecats. All these five T<eni<e certainly belong to but a 

 single species of tape- worm, and only present varieties of race 

 dependent on the influence of the varying circumstances to which 

 the young Taenice are exposed in the course of their further deve- 

 lopment, according as they have entered the digestive canal of a 

 man, a dog, a wolf, or any musteline carnivore. If we consider 

 the diagnoses of these five kinds of Taenice given by helmintho- 

 logists, we shall be convinced that there is not a single specific 

 mark of distinction to be found between them, and that the form 

 and arrangement of the circlet of hooks of these tapeworms are 

 entirely disregarded. If the heads of the five above-mentioned, 

 so-called species of tapeworm, with their circlet of hooks were 

 submitted to the inspection of the most experienced helmintho- 

 logist, without betraying their origin to him, I feel persuaded that 

 he would be perplexed in distinguishing those five species of Ttenice, 

 which only differ according to their various habitations. In the 

 genera Ligula, Schistocephalus, Tetrarhynchus, and Echinorhynchus, 



