CESTODA 283 



Since the most remote period another question has again and again occupied 

 the attention of naturalists, the question of the morphological nature that of the 

 INDIVIDUALITY OF THE TAPEWORM. The ancients, who were well acquainted 

 with the proglottids (Verities cucurbitani^/ that are frequently evacuated, were 

 of the opinion that the tapeworm originated through the union of these separate 

 proglottids, and this view was maintained until the end of the seventeenth 

 century. In 1683 Tyson discovered the head with the double circlet of hooks in 

 a large tapeworm of the dog ; Redi (1684) was also acquainted with the head and 

 the suckers of several Tasniae. Andry (1700) found the head of Tcenia saginata, and 

 Bonnet (1777) and Gleichen-Rusworm (1779) found the head of Dibothriocephalus 

 latus. Consequently most authors, on the ground of this discovery, considered the 

 tapeworm as a single animal, that maintains its hold in the intestine by means of the 

 head, and likewise feeds itself through it. The fact was recognized that there were 

 longitudinal canals running through the entire length of the worm, and it was thought 

 that these originated in the suckers, and that the entire apparatus was an intestine. 

 As, moreover, the segments form at the neck, and are cast off from the opposite 

 extremity, the tapeworm was also compared with the polyps, which were formerly 

 regarded as independent beings. 



Steenstrup, in his celebrated work on the alternation of generations (1841), was 

 the first to give another explanation. This has been elaborated still further by 

 van Beneden, v. Siebold and Leuckart, and until a few years ago all authorities 

 adopted his views. According to this view, the tapeworm is composed of numerous 

 individuals, something like a polyp colony, and, in addition to the proglottids the 

 sexual individuals which are usually present in large numbers there is ONE 

 individual of different structure, the scolex, which not only fastens the entire colony 

 to the intestine, but actually produces this colony from itself, and therefore is 

 present earlier than the proglottids. The scolex is a " nurse," which, though itself 

 produced by sexual means, increases asexually like a Scyphistoma polyp ; the 

 tapeworm chain has therefore been termed a strobila. Consequently the develop- 

 ment of the tapeworms was explained by an alternation of generations. In support 

 of this opinion it was demonstrated not only that the adult sexual creatures, the 

 proglottids, can separate from the colony and live independently for a time, but 

 that in certain Taenise, and especially in many Cestodes of the shark, the proglottids 

 detach themselves long before they have attained their ultimate size, and thus 

 separated continue to develop, grow and finally multiply ; the scolex also exhibits a 

 certain independence in so far as, though not, as a rule, capable of a free life, yet 

 it in some cases lives as a free being, partly on the surface of the body of marine 

 fishes and partly in the sea. With the more intimate knowledge of the develop- 

 ment of the cysticerci, the independent nature of the scolex was recognized. It is 

 formed by a budding of the bladder that has developed from the oncosphere, in 

 some cases (Ccenurus) in large numbers, in other cases (Echinococcus) only after 

 the parent cyst has developed several daughter cysts. Released from its mother 

 cyst and placed in suitable conditions, it goes on living, and gives rise at its 

 posterior end by budding to the strobila, the proglottids of which eventually become 

 sexual individuals. 



In order to make this clearer we will briefly summarize what takes place in 

 the jelly-fishes. 



By metamorphosis is meant a developmental change in the same individual, while 

 alternation of generations, or metagenesis, implies a stage in which reproduction of 

 individuals takes place by a process of budding or fission. This asexual reproduc- 

 tive stage alternates with the sexual mode of reproduction. Thus in the development 

 of the Scyphozoa (jelly-fishes) we have: 



(r) The fertilized egg cell divides regularly and forms a morula. 



