The edge hooks (Fig. 31) at first are located along the pe- 

 riphery of the disc whence they receive their name; however, as will be 

 stated later, in a number of species and groups they can also be located 

 on its central surface. The structure and sizes of edge hooks vary. In 

 the typical case the edge hook is divided into two basic parts, the first 

 is the parenthesis or sickle-shaped edge hook with a sharpened free end 



and with a handle extending from 

 the top end of the curved part of the 

 end hook in the shape of a differ- 

 ently arranged stem. For the most 

 part, a lateral growth which appears 

 as if it were constructed of the end 

 of the end hook and the beginning of 

 handle lies between these sections 

 or two parts. Among many forms 

 (for instance Polystomatidae and 

 Microcotylidae) there is a special 

 loop or structure near the end hook 

 which lies at the lower part of the 

 hook and through which it extends. 

 We will consider this formation in 

 connection with the fact that it plays 

 a specific role during the formation 

 of the attaching valves (see also 

 page 407). The number of edge hooks 

 hooks m various groups varies, 

 usually it fluctuates between 16 and 

 10, or to be more precise from 8 to 5 pairs for as a rule they are sym- 

 metrically located on the disc. In certain cases the number of edge hooks 

 is smaller in connection with the disappearance of part at the time of the 

 postembrynnic development (as for instance Microcotylidae, see page X.'X). 



Middle hooks (Fig. 32), as their name indicates, are primarily located in 

 the center of the attaching disc, and later in a number of forms (just as in orthogenesis, 

 also in phylogenesis) they can be displaced to the posterior end or the sides of the disc. 

 In a typical case the middle hook represents an elongated plate sharpened at one end 

 and forming two growths (continuations) on the opposite end, bent in one flat area in 

 such a way that its sharpened end lies much closer to the inner growth than to the outer 

 growth. The correlation between the growths in the basic parts and the point are very 

 different just as the size of the entire hook varies. Sometimes the middle hooks have 

 another form different from the typical one, but which is similar in form to the edge 

 hooks. In the latter case their origin is usually not clear, and it is possible that we are 

 dealing here with altered edge hooks which are not homologous to the middle hooks them- 

 selves (see chapter on embryology, page 101). Between the middle hooks, various 

 chitinous bars can be located (Fig, 33) which serve to link the middle hooks, and 

 also for the attachment of the musculature of the latter and for fastening p. 



Fig. 31. Marginal hooks: 

 A- -dactylogyrid type; 

 B - -gyrodactylid type; 

 C - -octocotylid type; and 

 D- -polystomatid type 

 (Schematically). 



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