23 1 DESCRIPTIONS OF PREPARATIONS. 



however well marked in the young Ligula, much less so in the sexual worm : so too 

 Triaenophorus. .It exists in the barren posterior region above-mentioned, but this 

 may be considered as arrested in growth. The setting free of the joints may well be 

 an adaptation, and is possibly due to the completed development of the embryoes 

 and consequent regressive metamorphosis of the genitalia. 



Other regressive changes appear to occur at the same period. Hamann states 

 that the nerves degenerate in the ripe joints of T. lineata : and Megnin contends 

 that the scolex may lose its hooks and its suckers, and may even atrophy away com- 

 pletely. See Journal de 1'Anat. et Physiol. xvii. 1881. Such changes may occur, 

 according to him, in T. serrata and T. solium. Donnadieu found that Ligula 

 sometimes undergoes digestion when its ripe ova are discharged. Facts such as 

 these show that no absolute conclusion can be based on the growth of joints after 

 their detachment in the tapeworms of some fish (supra]. 



As to the two supposed asexual generations, the proscolex and the scolex, the 

 question appears to turn on the following points : the asexual reproduction of the 

 proscolex as proscolex : the mode in which the scolex takes origin from the pro- 

 scolex : the character of the connection between proscolex and scolex : and the 

 apparent necessity for two hosts. 



The proscolex does not generally multiply itself asexually, but gemmation 

 takes place in the Staphylocystes of Villot, in Echinococcus (?), and in an Echino- 

 coccoid form discovered by Metschnikoff (cf. Leuckart, ' Parasiten,' i. p. 464) and 

 it is possible that an imperfect fission may sometimes occur, e. g. in Coenurus. 

 Such increase may be compared with the fission of a Trematode Sporocyst, e. g. of 

 Fasdola hepatica^ an undoubted representative of a generation, or with its gemma- 

 tion as in Leucochloridium : with the division of the embryo Lumbricus trapezoides, 

 an immature individual : with the formation of germs by the tail, i. e. an organ, of 

 the Cercaria-larva, Bucephalus pofymorphus, or of Cercaria cristata, according to the 

 observations of Ercolani : or with the separation of parts of the hydrocaulus or 

 stem which develope into individuals in a Campanularian, the Schizocladium of 

 Allmann. In other words, the occurrence of asexual reproduction does not neces- 

 sarily prove that the proscolex is an individual. 



The scolex is derived from a local thickening of a layer of cells, part of the 

 body-wall of the proscolex : but its muscular layers and its excretory system 

 become completely continuous with the corresponding structures of the proscolex. 

 This mode of origin is utterly unlike the way in which germs originate from the 

 walls of a Trematode Sporocyst or Redia, viz. by the growth and division of a cell 

 from the layer lining the coelome, or from the mass filling it at first (cf. Thomas, 

 Life History -of the Liver Fluke, Q. J. M. xxiii. 1883, p. 125, and figs, referred to) ; 

 nor does it resemble the mode in which buds are formed in the Metazoa with the 

 participation of all the germinal layers. With reference to this last point, however, 

 it must not be forgotten that the germinal layers are at no period distinct in the 

 Cestoda. It may be added that in a form discovered by Gruber (Z. A. i. 1878) in 

 Cyclops serrulatus, there is no apparent proscolex at all, and the scolex developes 

 without invagination. 



The proscolex and scolex may remain permanently attached to one another as 

 in Archigetes Sieboldi. The connection may only be severed when the first joints 

 are detached, as in many Phyllobothrians and Phyllacanthians which infest fish. 





