PISCES 



465 



are supplied with accessory horny developments. The genus 

 Gyrodactylus has been studied by Nordmann, Von Siebold, G. 

 Wagener, Yan Beneden, and especially by Wedl, who records 

 the following results : (a.) " Gyrodactylus is found on the gills 

 of fresh- water fishes under numerous specific forms, G. elegans 

 being also found by Creplin and 

 Siebold on the fins. Moreover, as 

 I have found nearly every species 

 of fish supporting a particular gyro- 

 dactyle representative, it would seem 

 that each finny creature supplies its 

 own Gyrodactylus. Sometimes two 

 of them are parasitic upon the same 

 gill, being frequently associated with 

 Trichodina, as well as with the still 

 unintelligible Psorospermia. (b.) 

 The clasping apparatus at the pos- 

 terior end of the body must in an 

 animal so soft and constantly ex- 

 posed to the passage of regular 

 currents be comparatively strongly 

 developed and accommodated to the 

 peculiar dwelling-places, and pro- 

 bably the varying character of the 

 latter supplies a reason why there 

 should be so great a difference in 

 the mechanism of the hooks belong- 

 ing to the disk, (c.) The hooked 

 apparatus affords a very valuable 

 and mathematically precise means 

 of diagnosis in the determination 



Of Species. This differentiation FIG. 7?. Gyrodactylus elegans, containing 



. an embryo, a, a, (Esophagus; a, testis ; 



inaV be accomplished by Observing h, h, sucker ; i, i, large hooks ; I, spines. 



J J Magnified. After Van Benedeu. 



whether there are two or four large 



hooks ; whether there be one or two connecting portions, and 

 by noticing their several forms and relations to one another ; 

 and whether, again, there are booklets or not, remarking in the 

 first instance their position, form, distribution, and so forth. 

 (d.) The integument is sometimes wrinkled transversely, at 

 other times appearing to be smooth. (e.) The muscular 

 apparatus is, in certain cases, very strongly developed. In the 

 majority of instances special muscles are inserted into the 



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