D.DImm 



not differentiated and has a common contour with the body representing 

 its direct and gradually narrowing end. The internal organization of the 

 larva resembles that of the adult worms, its anterior end is equipped 

 with two growths into which head glands open. At first these growths are only 

 weakly noticeable. Two pairs of pigmented little eyes of very large size 

 are located between them and the pharynx. The posterior pair is larger 

 than the anterior and is equipped with larger light-refracting lenses. 

 There are weakly developed nervous and excretory systems. The digestive 

 system is represented by a well-developed small pharynx and in relation 

 to the size of the pharynx, a disproportionately small intestine which has 

 the appearance of a ring. The chitinous armature of the attaching disc is 

 represented by very frail small edge hooks, the nunnber of which is the 



same as among the adult forms. 

 These edge hooks which as yet do 

 not have the final shape are located 

 in the posterior part of the body and 

 lie there in groups which are more 

 or less parallel to the axis of the 

 body of the larva, motionless and 

 not extending outside. After a certain 

 interval of time, somewhat before 

 the finding of the host, considerable 

 changes take place in the behavior 

 and structure of the larva. The larva 

 which has just emerged from the egg 

 possesses a positive phototaxis which 

 later changes to negative. The 

 attaching disc of the larva begins to 

 delineate itself, acquires the typical 

 form and relationship to the body; it 

 stands at a certain angle to the body, 

 becomes round and at that time is 

 much thinner than the rest of the body. 

 At the same time, the edge hooks, 

 so to speak, descend toward the edges 

 of the discs and occupy their final 

 places, they cut through to the outside terminally and at times begin to 

 move in a lively fashion. The larva begins to stop near different under- 

 water objects as if feeling them with its anterior end (at that time the head 

 lobes are already distinctly visible), it often contracts and attempts to 

 attach itself. This period normally terminates in the larva finding its host 

 and attaching itself to its gills, fins or skin. When it does not find a host, 

 it descends to the bottom and perishes rather quickly. Among representa- 

 tives of the genus, the period of the free -swimming larva extends from 4 

 to 20 hours and the period during which the laiva is capable of infecting a 

 host is considerably shorter- -does not exceed 5 hours. 



p. 142 



Fig. 138. Dactylogyrus vastator 

 Nybelin, larvae in different stages 

 of contraction. 



150 



