CESTODA. 663 



cell alone. It has been found in those instances which have been accurately 

 investigated, that the superficial cells of the embryo give origin successively 

 to two envelopes (i) an outer, which is very delicate in Ligula and other 

 Pseudophyllidea, but in Taeniae constitutes an albuminogenous layer ; 

 (2) an inner, which loses its cellular character and is ciliated in Ligula, 

 Triaenophorus , Schistocephalus, and some species of Bothriocephahis, but is 

 non-ciliated and capable of creeping movements in other species and in 

 certain Taeniae inhabiting water birds and fresh-water fish ; whilst in most 

 other Taeniae it constitutes the chitinogenous layer and is transformed 

 into a firm chitinoid embryonic shell (cf. p. 227-8 ante). When the shell 

 is non-resistent it increases in size during the evolution of the embryo and 

 eventually disappears. 



The chitinoid shell is lost when the Taenian embryo enters the stomach 

 of its first host. The soft ciliated or non-ciliated coat of the Pseudophyll- 

 idea, &c. is stripped off after a time, a gap previously appearing between 

 it and the contained embryo ; it is possible, however, that this occurrence 

 takes place normally only after the entry of the embryo into its first host. 

 The embryo itself is cellular, and provided as a rule with six hooks 

 arranged in pairs close together at one pole. The number of hooks may 

 be increased 1 . The muscles which move these hooks are sometimes visible 

 as delicate lines. The embryo itself is composed of cells, among which a 

 superficial set may be distinguished from a more central, the former 

 appearing to grow round the latter 2 . It is possible that the former may 

 represent an epiblast, the latter a hypoblast. After its entry into the first 

 host (p. 656) the embryo or proscolex grows and its tissues become 

 differentiated. The account given of T. serrata p. 230 may be considered 

 as typical. The adventitious connective tissue cyst formed by the meta- 

 morphosed lymph cells of the host is not (?) present when the first host is a 

 non-vertebrate. The size attained by the proscolex varies much. When 

 its central cells liquefy, as in many Taeniae, and it is large, it is more or 

 less globular in shape ; when they do not liquefy and it is large, it assumes 

 an elongated shape, e. g. in Tetrarhynchus. It sometimes grows into an 

 irregular or branched form, due apparently to the influence of the tissues 

 surrounding, e. g. the racemose form of Cysticercus cellulosae from the base 

 of the human brain. It may multiply asexually as in the formation of 

 brood capsules or daughter vesicles in Echinococcus, in Staphylocystis and 

 Urocystis which inhabit the Myriapod Glomeris limbatus. It gives origin 

 to a head and neck = scolex, except in Echinococcus, in which the scolices 

 originate from brood-capsules only, or the proscolex and its daugh'ter- 



1 Cf. Leuckart, Parasiten (ed. 2), pp. 417-18; Hamann, Z. W. Z. xlii. p. 728. The entire 

 absence of hooks in some cases, asserted by J. P. van Beneden (Vers Intestinaux, p. 237), is extremely 

 doubtful, and in certain instances, e. g. Ligula, Bothriocephahis, is known to be incorrect. 



2 Cf. E. van Beneden, Archives de Biol. ii. p. 198 ; Hamann, Z. W. Z. xlii. p. 728. 



