posterior two-thirds of the attaching disc. The cilia are relatively long 

 and form from a special epithelium which is clearly visible in live samples. 

 The loss of cilia and ciliary epithelium takes place simultaneously. The 

 attaching disc is folded during the time of the swimming of the larva; it 

 is equipped with middle hooks characteristic for all adult individuals. 

 These hooks lie longitudinally in such a 'vay that their curved ends are 

 located mesially and are visible on the ventral side. The edges of the 

 folded little sucker lie ventrally from the middle hooks and are equipped 

 with lateral hooks. The latter are of equal length, approximately 0.01 mm 

 in length. In live individuals the cephalic attaching organs are very mobile, 

 they even attach themselves to the container in which the larvae are located, 

 and even draw up the entire body behind them. The loss of the ciliary 

 epithelium begins soon after the appearance of this ability. Usually the 

 ciliary covering of the anterior and posterior zones are lost first. The 

 attaching disc unfolds at the same time as the shedding of the epithelium 

 (Fig. 198, A). When it has completely unfolded the middle hooks turn 

 along their long axes and their curved ends protrude outside whereas the 

 lateral hooks are disposed radially along the edge of the attaching disc. 

 At that time the attaching disc already begins to function and attaches to the 

 container. When the larva finds a host, in the beginning it probably attaches 

 by means of the head organs and only afterwards by means of the disc, 

 more precisely by means of its 14 edge hooks and perhaps by means of the 

 posterior pair of the middle ones. After the attachment of the larva (Fig. 

 198, B) the first noticeable morphological changes are expressed in the 

 transformation of the attaching head organs into the final head suckers. 

 Further, the pharynx which is rounded in the larva acquires the lobed 

 shape very characteristic for the adults. The intestinal trunks begin to 

 put out lateral growths. The diameter of the attaching disc is relatively 

 large in relation to the body. Its middle hooks increase in size and change 

 their form, whereas the lateral do not change and do not grow. Because 

 of this we can deduce that they do not have any significance in the adult 

 form. Further development is expressed mainly in the development of 

 the digestive system and the increase of the excretory system and par- 

 ticularly in the development of the sex system. The size of the eyes in 

 the larva and among the adult is approximately the same. Only their 

 location which is trapezoid-shaped is different among the adults. One 

 must also note that the growth of the (m.iddle hooks of the, nobis) attaching 

 disc proceeds unequally so that at the end of the period of growth the size p. 180 

 of the middle and anterior pairs is nnore than six times larger than of the 

 sanne ones among young worms, whereas the posterior pair increases only 

 three times" (Jahn and Kuhn, 1932), 



The free-swimming larvae of^ derzhavini, which have just 

 emerged from the eggs on the 10th day after their deposition are very 

 similar to the larvae of B. melleni, but differ in a number of peculiarities. 

 Their sizes are somewhat larger and the correlation of their parts is 

 different. The length of the larvae is about 0. 26 mm and the width in the middle 



194 



