214 
LIFE, IN ITS INTERMEDIATE FORMS. 
hand, the residence of another group, the Epizoa, of 
which wo are speaking. 
Though these two groups of parasitic animals aro 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 must be borne in mind that our lines of demarcation 
are artificial, though, for perspicuity’s sake, we must 
draw them somewhere. 
One of tho 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 tlicir prey, and various are the mechanical contrivances 
which serve this purpose. There is a minute Worm 
( Gyrodactylus ) 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- 
