TAPEWORM IN CYSTIC STAGE. 233 



Or the scolex may separate from the proscolex, and either enter an intermediate 

 host, e. g. some species of Tetrarhynchus, or even in rare instances remain free (cf. 

 Moniez, Travaux, &c. iii. i, p. 142). Finally, the proscolex may be digested in the 

 stomach of the second host, but so also are the barren joints formed by the scolex 

 of Cysticercus fasciolaris (= Taenia crassicollis] while still encapsuled in the Mouse. 

 There is evidently much variety observable with reference to the character of the 

 connection between the two structures. 



It is a general fact that two hosts are necessary for the evolution of a Cestode. 

 Archigetes however is an example of the sufficiency of a single host. Ligula attains 

 immature sexuality in a fish, its first host, so that a few days' sojourn in its second 

 host, a water-bird, brings about sexual maturity. Taenia solium may exist in the 

 flesh of man in the Cysticercus-stage, in the alimentary canal as a Tapeworm ; and 

 it has been shown by Riehm and Leuckart that the scolex of C. pisiformis may de- 

 velope into a jointed and sexless worm in the intestines of the Rabbit, but whether 

 it would ever attain sexual maturity under these conditions is doubtful. That the 

 proscolex may develope in an alimentary canal is proved by P. J. Van Beneden's 

 discovery of proscolices with scolices in all stages of growth in the intestine of the 

 Lump-fish (Cyclopterus]. The same thing possibly occurs also with other Tape- 

 worms inhabiting fish. Why there should be two hosts is a most obscure question. 

 Leuckart appears to think that there is a physiological connection between the two, 

 inasmuch as one is the prey of the other. Moniez has broached the idea that a 

 change from one to another host is, in these animals, which are so completely 

 parasitic and therefore dependent, necessary to maintain their vigour. 



The facts adduced certainly weaken the generally adopted view as to the 

 existence of an Alternation of Generations in the Cestoda. There can be little 

 doubt that the strobila is to be regarded as a single organism, and the same state- 

 ment is probably true of the scolex and proscolex 1 . It is evident that the Cestoda 

 are profoundly modified by parasitism, not only structurally but developmentally. 

 The study of lower forms in the class may throw more light on the many obscure 

 points connected with its evolution, but unfortunately there are great intrinsic diffi- 

 culties in the way of such an investigation. 



Development of scolex and life-histories. Leuckart, op. cit., Moniez, Les 

 Cysticerques, and P. J. van Beneden, Vers Intestinaux and Vers Cestoides ; see 

 p. 228, ante. 



Archigetes Sieboldi, Leuckart, Z. W. Z. xxx. (Suppl.), 1878 ; Gruber, Z. A. iv. 1881. 



A Gemmating proscolex, Staphylocystis, Villot,' T^nias des Musaraignes, A. Sc. 

 N. (6), viii. 1879; Urocystis, Id. A. N. H. (5), vii. 1881. 



On the question of Alternation of Generations in Cestoda, see Riehm, Zeitschr. 

 f. d. ges. Naturw. (Giebel), 54, 1881, p. 590; Moniez, Les Cysticerques, op. cit. 

 pp. 135-154 ; Leuckart, Die Parasiten (ed. 2), pp. 488-490 ; P. J. van Beneden, Vers 

 Intestinaux, p. 242 et seqq. Comparison with Trematoda. Van Beneden, ibid, 

 p. 251 et seqq. 



Parasitism. P. J. van Beneden, Animal Parasites and Messmates, Internat. 

 Series, xix. 1876. 



1 Moniez regards the scolex simply as an organ of adhesion. To this view Niemiec opposes 

 the character and degree of development attained by the central nervous system which is always 

 lodged in it. 



