(clamps) and equipped with chitinous apparatuses of different complexity. 

 In the simplest cases the armature of these formations represents 

 strongly developed edge hooks and in the most complex case--a system 

 of chitinous parts forming the two hard valves of the pinching structure 

 (clamps, nobis). The arnnature of the larvae is partially preserved on 

 the disc of the adult animals or fully disappears through metamorphosis 

 or by means of shedding. The cephalic end has 3 groups of glands. The 

 buccal opening is subterminal and bears a w^ell-developed sucker, or there 

 are 2 special suckers in the buccal cavity which are not homologous to 

 the suckers of the cephalic end of Polyonchoinea. As a rule, the eyes are 

 absent among mature worms, very rarely they are preserved during the 

 entire life (Diclybothrium). 



Parasites of Elasmobranchii, Teleostomi (the large majority) 

 and Holocephali. As an exception they are encountered on parasitic 

 Isopoda. Thus, the representatives of this class have a narrower circle 

 of hosts than Polyonchoinea; however, they are discovered on all groups 

 of fishes on which the latter are also found. 



The subclass comprises families relating to Polyopisthocotylea, 

 according to the system of Price and Sproston, with the exception of 

 Polystomatidae and Sphyranuridae which were ascribed to the preceding 

 subclass on the basis of the data on their development and morphology. 



The families which enter into the subclass are grouped on the 

 basis of the structure of the attaching apparatus which has a tendency to 

 greater and greater specialization of the ability to attach itself only to the 

 gill filaments of a particular group of hosts. There are three main directions 

 of change: 1) with the preservation and development of chitinous hooks in 

 all stages of ontogenesis, the development of clamp-shaped suckers; 

 Z) with the preservation of chitinous hooks without further development, 

 the appearance of supporting elements in the clamp-shaped suckers; and 

 3) with the preservation or disappearance during the early stages of ontogenesis 

 of chitinous hooks, the development of a larger or smaller number of real 

 clamps bearing a complex system of supporting elements. For the last 

 trend the appearance of asymmetry of the attaching apparatus and often of 

 asymmetry and mirrorness of the attaching clamps themselves on the right 

 and left sides is characteristic (clamps asymmetrical but repeated on both 

 sides of the disk like mirror images, nobis ). The peculiarities of the 

 evolutionary development, which appears as a result of greater and greater 

 adaptation to their hosts, lead very quickly to a complete or almost complete 

 immobility of the representatives of the given suborder in contrast to what 

 takes place in a majority of Polyonchoinea (see page 75 ). Just as in the 

 preceding suborder we also observe a nunnber of cases of converging p. 403 



appearance of similar peculiarities here, the consideration of which is 

 indispensable in the analyses of the true phylogenetic relations. 



482 



