After attaching itself to the body of its host the larva throws off 

 the ciliary covering and begins to feed and grow at an accelerated rate. 

 During the growth of the body and parallel to it, takes place the growth of 

 the nervous, the excretory, and the digestive systems which gradually 

 acquire their final form. Very quickly after the attachment, the two head 

 growths are supplemented by two more and the head end also acquires its 

 final structure. The eyes of Dactylogyrus either do not grow at all from 

 the moment of emergence from the egg, or if they grow they do so very 

 slowly. Among many forms the eyes degenerate, dividing into separate 

 pigmented granules which remain near the anterior edge of the pharynx 

 during the entire life. 



5 5) Q) S 3 



Fig. 139. The diagram of the 

 development of edge hooks in 

 Dactylogyrus. 



The development of the attaching apparatus deserves undivided 

 attention. The edge hooks of the free -swimming larva (Fig. 139) have an 

 already well-expressed division into two parts: the terminal little hook 



and the handle. The terminal little 

 hook of the edge hook distinguishes 

 itself from that of the adult form only 

 by somewhat more general outlines 

 and thickness (at first among a 

 number of species it somewhat thickens 

 proportionately to the growth of the 

 entire edge hook); the handle of the 

 little edge hook is in the shape of a 

 little stick, sometimes with a small 

 widening at the free end. When this 

 widening occurs, its length (from 

 the terminal hook to this widening) corresponds to the length of the basal 

 part of the handle of a fully-developed edge hook. Subsequent growth of 

 the handle takes place by way of the accretion of newer parts onto its free 

 end until the lateral hook reaches the final dinaension and form. The stem 

 of the larval edge hook somewhat thickens during the time of its subse- 

 quent development just as in the terminal little hooks. The tempo of growth 

 of the edge hooks among different types of Dactylogyrus varies and the 

 development of hooks of separate pairs takes place unequally: some of 

 them reach larger sizes and have sometimes somewhat different shapes 

 then others (Fig. 140). For the most part, the second, third and fourth 

 pairs acquire larger sizes, whereas the sixth and seventh are the smallest. 

 In such a fashion, the accretion of new parts while preserving shapes and 

 linear nneasurements of those already existing is characteristic for the 

 development of the edge hooks. This phenomenon is so typical it is almost 

 always possible to indicate with a great degree of precision the sizes of 

 the hooks of the free -swimming larva from the sizes of the edge hooks of 

 the adult animal. One must note that the development of edge hooks of 

 D. anchoratus (Dujardin) is described quite correctly in the work of 

 Kulwiec (1927), whereas the figures of the development of edge hooks of 

 this species and also of D. vastator Nybelin and D. macrocanthus Wegener 



p. 143 



151 



