214 



LIFE, IN ITS INTERMEDIATE FORMS. 



band, the residence of another group, the Epizoa, of 



which we are speaking. 



Though these two groups of parasitic animals are very 

 diverse in zoological rank, or, in other words, in the de- 

 gree of complexity which their structure exhibits, they 

 merge into each other by imperceptible gradations, so 

 that there are some intermediate forms (as there almost 

 always are on the confines of great groups), which it is 

 very difficult to arrange in either Class, because this 

 would involve their violent separation from near kindred. 

 It mast be borne in mind that our lines of demarcation 

 are artificial, though, for perspicuity's sake, we must 

 draw them somewhere. 



One of the most interesting points in the economy of 

 these creatures is the variety which is displayed in their 

 armature. Deprived, for the most part, of limbs, or 

 having these members when present strangely disguised, 

 it was necessary to their existence that they should be 

 furnished with some means of affixing themselves firmly 

 to their prey, and various are the mechanical contrivances 

 which serve this purpose. There is a minute Worm 

 (pyrodactylus) which lives upon the gills of certain species 

 of the Carp tribe, whose adhering disk, when viewed be- 

 neath the microscope, is most formidable to behold. It 

 is armed all around its circumference with sharp curved 

 hooks, while its centre is provided with a pair of much 

 larger hooks, all intended to be plunged deeply into the 

 flesh of the unfortunate fish, while the blood is sucked at 

 leisure. In Caligus, a creature a hundred times as large 

 as that just named, found on various marine fishes, the 

 object is effected by an array of hooked fangs and pincer- 



