In the latter case and cases similar to it, it appears that the basic role is 

 played by the cavity which is created as a result of the contraction of the muscles 

 (in the posterior attaching apparatus, nobis) and which has less pressure in- 

 side than outside. This is substantiated on the one hand by the swelling 

 which remains on the body of the host (after the worm detaches or is de- 

 tached, nobis ) similar to the ones resulting from medical jars (suction jars, 

 nobis ) and on the other hand by the fact that if a very thin capillary, which 

 allows passage for the water from the outside, is placed under the disc of 

 the attached worm, there is little difficulty in removing the wornn,and in 

 most cases it will fall off itself. 



^•'^ 



j2^^=^^ 



Certain sections of the dis- 

 sertation of N. A. Izumova (1953) 

 were dedicated to the questions of 

 behavior among monogenetic trema- 

 todes on the gills of their hosts. 

 While studying the influence of the 

 oxygen content of the water on 

 Dactylogyrus solidus Achmerow 

 and D. vastator Nybelin, she suc- 

 ceeded in showing that the change 

 in the quantity of oxygen in the con- 

 tainer leads to a change in the lo- 

 cation of the first of these types. 

 Thus, with a decrease in the oxygen 

 content of the water D. solidus 

 actively move to the ends of the 

 filaments of the first and fourth 

 gill arches concentrating on their 

 ventral sections --places of the best 

 aeration and conversly with an in- p. 77 

 crease in the oxygen content the 

 worms return to the places of their 

 original location, that is, to the middle and lower parts of the filaments of the 

 second and third gill arches. The data of Izumova show that this is connected 

 with the conditions of aeration of the different sections of the gills. As re- 

 gards D. vastator, the change of oxygen supply does not cause a change in the 

 location of the worms; apparently these worms are much less demanding of 

 conditions of aeration. 



The species which are not capable of motion because of special 

 arrangements of attaching apparatus, are encountered nnainly in the highest 

 Monogenoidea, but they are also present among the lowest. Thus, Diplec- 

 tanum similis Bychowsky, which parasitizes near the bases of the gill fila- 

 ments of Corvina nigra Salv. , has such a distribution and arrangement of 

 connecting pieces of the middle hooks of the attaching discs that it is deprived 

 of the possibility of active transfer (movement, nobis) and at best can only 



Fig. 109. Nitzschia sturionis (Abild- 

 gaard), locomotion of the worms along 

 a flat surface. Sketches made on the 

 Island of Sara (Caspian Sea) from the 

 worms discovered in the buccal cavity 

 of Huso huso (L. ). 



74 



