326 DESCRIPTION OF THE PLATES. 



piercing the tissues, they have an antero-posterior movement. The two 

 pairs of spines again on the right and left are symmetrical with each other. 

 These two pairs move in piercing the tissues much as the fore-limbs do in 

 swimming. The wound made is, on account of the small size of the em- 

 bryo, invisible ; and it has been incorrectly supposed that the embryos 

 find their way into the liver by way of the bile-ducts. The six spines 

 are recognisable up to a certain period, though dislocated from their position, 

 in the more or less distended, cystic or cysticercoid vesicle, into which the 

 proscolex may expand when it reaches its place of lodgment in the tissues. 

 In Cysticerctis Arionis the hooks are retained in situ, see Leuckart, op. cit. 

 Fig. 209, p. 459. Figures of embryos and the history of their migrations 

 are given in the works of Van Beneden and Leuckart referred to. 



FIG. 5. Cystic stage in the development of Many-headed Bladder-worm, Coenurus cerebralis, 

 after Van Beneden, I.e., PL xxvi. Fig. 31. 



THE hexacanth embryo, figured at 4, has grown greatly after coming 

 to rest in the organ, ordinarily the brain of a sheep, to which it is carried 

 by the blood-current. The six hooks are observed to be dislocated at #, 

 and a number of ' scolices,' the heads of as many future Tapeworms, are 

 developed upon one of the poles of the vesicle. The way in which these 

 heads are formed in the Taeniae is detailed on p. 230. There are 300 to 

 400 heads in Coenurus. Each head at first points inwards towards the 

 interior of the parent cyst ; but by the contractions of the muscular layers 

 of the cyst, as also by those of the intrinsic muscles of the head itself, it 

 may point either outwards or inwards. For the changes that occur when 

 the cystic animal is swallowed by the host, see p. 230 already quoted. 



a. Wall of embryonic dilated cyst. 



b. Hooks of embryo dislocated by its growth. 

 c I. Scolex fully protruded. 



c 2. Scolex half protruded. 



d. Scolex as developed pointing inwards and its tubular body in communi- 

 cation therefore, not with the parent cyst, but with the cavity of the 

 adventitious cyst thrown round the entire organism by the irritated 

 tissues of its host. 



For history of Coenurus cerebralis, see Van Beneden, /. c., p. 146 ; Cobbold, 

 Entozoa, 1864, p. 116 seqq. ; Id. Parasites, 1879, p. 333 ; Gamgee, Report on the 

 Parasitic Diseases of Quadrupeds used as Food, Med. Officers' Privy Council Office 

 Report, v. 1862 ; Thudichum, ibid., vii. 1865. 



FIG. 6. Hydra viridis, with reproductive organs ; after Greene, Manual of Coelenterata, 

 1861, p. 24. 



THE animal is drawn, attached by its ' adhesive ' or ' pedal ' disc to a 



