Shape of the Body. Bilateral symmetry is the most common, 

 with the longitudinal axis of the body with a narrowed and rounded an- 

 terior end, and an adhesive disc on the posterior end which is more-or- 

 less separated from the rest of the body. The ratio between the longi- 

 tudinal and transverse axis fluctuates significantly in different species. 

 For instance, certain individuals of 

 Tristoma cdccineum Cuvier (Fig. 4) 

 may be as wide as they are long, and 

 the above mentioned Squalonchocotyle 

 somniosi (Causey) has a length which 

 exceeds the width by 11 times. The 

 most common type is 3-4 times longer 

 than wide, as for instance, with the 

 majority of Dactylogyridae (Fig. 5). 

 Along with these bilaterally sym- 

 metrical species we also find asym- 

 metrical species such as Vallisia 

 striata, Perugia and Parona (Fig. 6), 

 which shows lateral growth in the 

 middle of its body that divides the 

 body into two separate sections. The 

 posterior section is strongly curved 

 and unequally developed in relation 

 to the longitudinal axis. Asymmetry 

 of such a type is undoubtedly a second- 

 ary phenomenon. There are also other 

 types of asymmetry, also of secondary 

 origin, but linked to a special type of 

 evolution of the adhesive apparatus. 

 The latter, in a series of cases, develops 

 only from one side of the body, thus 

 resulting in asymmetry, nevertheless 

 the internal structures of the body re - 

 main bilaterally symmetrical. In 

 other cases, which are superficially 

 similar but different in nature, asym- 

 metry results from the displacement 

 of the symmetrical adhesive disc to- 

 ward one side of the body of the animal. 

 Examples of asymmetry of the first 

 type can be seen in representatives of 

 the species of Gastrocotyle (Fig. 7) and 

 the second type in the species of Axine 

 (Figs. 8 & 9). 



p. 13 



Fig. 2. Squalonchocotyle 

 borealis (Beneden), adult 

 worm from the gills of 

 Somniosus microcephalus 

 (Bl. and Schn. ) near the 

 banks of Murman (Barents 

 Sea) 



