T. B. ROSSETER OX EXPERIMENTAL INFECTION OF DUCKS. 403 



localities in the intestine, demonstrated. A comparison of the 

 hooks of each of these species of tapeworms, Figs. 2, 6, 12, with 

 those of the Cysticercus, Figs. 4, 8, 14, which are in each 

 instance a facsimile of my mounted specimens, place it beyond 

 a doubt, more especially in this instance, as they (the hooks of 

 the mature scolices) are the product of direct infestation, that 

 they are not only similar, but, in conjunction with the em- 

 bryonic rostellum which foreshadows that in the mature scolex, 

 an exact counterpart of each other ; and thus their identity 

 is proved. 



The itinerary or life-cycle of these tapeworms is governed by 

 what Steenstrup denominated the " law of alternation of genera- 

 tions," that is to say the Cysticercoid stage, which is evolved from 

 the Oncosphere, has to be " nursed," and its nurse must become 

 the food of the vertebrate, which is, or will be, the final host of 

 the specific tapeworm ; and in which the Taenia will be perfected 

 and propagate its species. This is in the regular course of nature, 

 and is performed in a natural manner and governed by a natural 

 law ; and if this law is subverted by the nurse becoming the food 

 of a creature other than the specified final host, as in the above 

 case of Cysticercus gracilis in the perch and stickleback, of the 

 species of tapeworm to which the Cysticercoid is affiliated ; it (the 

 Cysticercoid) has then wandered or strayed ; and, following the 

 course of nature under these circumstances, it perishes. Thus, if 

 we find a Cysticercus in which a doubt exists as to its alfinity 

 with any known tapeworm which has been described or figured 

 by a previous investigator, and we wish to feed experimentally 

 with the Cysticercoid in question so as to produce the mature 

 tapeworm and elucidate the subject as to its probable final host, 

 then we have to consider the environment of its nurse and thus 

 form our judgment of the class of creature it (the nurse) would in 

 all probability fall a prey to as its food, and thus cause, in its 

 intestine, the final development of the parasite of which it (the 

 nurse) was but the intermediate stage. It is not always the case 

 that the selection of the final host, for experimental infestation, 

 is facilitated as it was in these three instances, because their 

 Taenia was known, and the Cysticercoids, with their nurse, were 

 plentiful in the pond, and the artificial infestation of the ducks 

 with the infected crustaceans was prolific in its action, as the 

 results proved. It is when the percentage is small, or, as in 



