TAPEWORM IN CYSTIC STAGE. 231 



The proscolex in T. serrata gives origin to a single scolex, and the resulting 

 organism is therefore termed Cysticercus. When it produces a number of scolices, 

 the resulting organism is a Coenurus, e. g. C. cerebralis of the Sheep, the cause of the 

 disease known as ' sturdy,' ' gid,' or ' staggers ; ' and when scolices are produced not 

 directly by the proscolex, but indirectly from ' brood-capsules,' which originate from 

 the proscolex in the first instance and remain attached to it, the organism is an 

 Echinococcus. 



For Figure of Coenurus, see PI. xiv. fig. 5. 



The cyst in which the Cysticercus lies is formed by the irritated tissues of its 

 host. Its inner surface is covered by a layer of epithelioid cells : its walls are com- 

 posed of connective tissue and contain blood-vessels. The whole structure is 

 produced, apparently, by the metamorphosis of lymph-cells. 



The life-history of T. serrata and of other Cestoda is generally supposed to in- 

 clude three successive generations : two asexual, the proscolex and the scolex ; one 

 sexual with numerous individuals, the proglottides. The last-named are supposed 

 to be produced, one after another, by posterior gemmation of the scolex, from which 

 they are detached in many instances either singly or in groups. Many interesting 

 features of resemblance between a fully-formed proglottis and a Trematode have 

 been pointed out by P. J. Van Beneden (cf. Vers Intestinaux, p. 251, and PI. 

 xxvii). But the facts do not appear to necessitate the view that the proglottis is an 

 individual : and Riehm especially has drawn attention to certain particulars. The 

 setting free of a proglottis may be paralleled with the setting free of the hecto- 

 cotylised or sexual arm of many male Cephalopoda : the formation of new pro- 

 glottides with the re-development of this arm, or to the building up of a complete 

 worm from two or three somites, as in the Oligochaete Lumbriculus variegatus. Con- 

 sidered as an organism, a proglottis is unable to maintain its own existence : it has 

 no organs of adhesion, and as a rule it is placed by its detachment under destructive 

 influences. If however it remains under favouring circumstances, i.e. within the 

 intestine, it may increase in size (Vers Intestinaux, p. 249 ; Vers Cestoides, pp. 123, 

 143), just as the fragments of a Nemertean may continue to live and mature 

 sexual products. However, both Leuckart and Van Beneden are inclined to regard 

 this fact as decisive for the zooid-nature of a proglottis. But it is not clear from 

 Van Beneden's account to what the increase is due ; nor perhaps is it absolutely 

 certain that dissection of the host may not cause separation of the joints from the 

 strobila. Such separation often occurs with the slightest disturbance (cf. Vers Ces- 

 toides, p. 139). Turning to anatomy ; though the deep longitudinal muscles do not 

 extend across the interval between successive joints, and are interrupted even in 

 Ligula, yet there are longitudinal muscles which do so (Riehm); the nervous 

 system is continuous throughout the worm : so too the excretory system, but the 

 transverse anastomoses of the Taeniae do not exist in other forms ; the ovaries of 

 Ligula appear to be continuous (Moniez), and discharge of the proscolices from 

 the uterus may take place long before the joints are detached, e.g. in Bothrio- 

 cephalidae and Ligulidae. Moreover the primitive terminal joints may remain barren 

 or develope sexual products at a relatively late period. The degree to which seg- 

 mentation is marked externally is variable : and in Triaenophorus and Ligula is 

 scarcely discernible, much less so in the latter than in the former. The formation 

 of joints is usually held to depend on the evolution of the sexual organs. It is 



