14 ANIMAL PAEASITES. 



This very point-cover lias its own history, so many erroneous inter- 

 pretations has it received. It has usually been regarded as a 

 hook, which may easily happen, when the point is bent back and 

 the mechanism of its movement is unknown. Hence all authors had 

 hitherto described the point-covers as " the small hooks of the 

 Pentastoma." According as the authors saw all the four point- 

 covers, or only some of them, they speak of two, three, or four 

 small hooks. The mistake is explained as soon as one sees a 

 living animal. The hooks swing with their base in the fork, and 

 when in movement, press back the lobes of skin with the hinder- 

 most point on the lower extremity of the true convexity of the 

 hook. At the same time the point of the hook escapes downwards 

 from the cavity of the point-cover. If the hook returns again to 

 a state of repose, the lobes of the ventral skin again apply them- 

 selves more closely to the whole hinder surface of the hook, the 

 point glides back into the cavity of the little point-cover, and is 

 completely covered by it. With this mechanism the chitinous 

 thread or ridge in the middle between the lobes of skin would be 

 very well adapted to facilitate the play of the hooks, by stiffening 

 the lobes, and, as it were, forming a firm channel for them. 



The measurements of the hooks are as follows, according to 

 Zenker : 



Distance between the point of the hook (a) and the foremost 

 point of the base (b) swinging in the fork, 0-042'" = 0'095 mill. 



Distance between the hook-point (a) and the hinder extremity 

 of the emarginate basal portion of the hook (c) 0-083'"=0-188 

 mill. 



Distance from b c = 0'055'" == O124 mill. 



Distance between the point b, and the hinder convex extremity 

 of the hook, in a straight line, 0'059 //x = 0-133 mill. 



This parasite is not very rare amongst us, as Zenker found it 

 thirty times in two hundred dissections. 



Second Family SIMONIDA (Vogt). 



Corpus vermiculare, delate proficiscenle diminutum ; cephalo- 

 thorace laliore et molli ; pedes breves Iruncique, mediano aculeo 

 majore armati, in statu immaluriore pedibus 6, in maluriore 8 ; 

 organa manducaloria roslellum parvulum medianum, duabus laminis 

 gladiformibus acutis armalum, palpisque 2 brevibus, duos articulos 

 exhibentibus conicisque mstructum prcebenlia. 



