109 
and Mrgasilus became simple. Further change in form 
and habit continued as the various appendages, through 
constant rest, degenerated. ‘The animal became in con- 
sequence more and more dependent on its host for food. 
Such changes extending over a long period of time, 
have apparently resulted in such a form as Lernea, 
Some Zoologists divide the fish parasites into blood- 
suckers and mucus-eaters, on account of the apparent 
presence or absence of blood in the alimentary canal. It 
is doubtful if such a division is really satisfactory. The 
probability is that they are all blood-suckers in different 
degree, and that the presence of blood is only obvious 
because certain organs are absent. Lamargus muricatus, 
one of the Caligida, appears to make excavations into the 
skin of its host, Orthagoriscus mola (the short sun-fish). 
Several individuals are usually found in each excavation.* 
No obvious appearance of blood can be observed even in 
these parasites. 
One or two parasites on a fish may not be hurtful, but 
when the numbers increase they probably have an irri- 
tating effect, and finally, when they remain in one position 
for some time, the skin and tissues become lacerated. 
Consequently even such external parasites as have been 
regarded as harmless mucus-eaters may really have an 
injurious effect upon the fish. 
There is much opportunity for investigating the 
internal structures of the various families of fish parasites. 
The most of the literature hitherto published deals with 
the external characters only. 
The specimens necessary for the work connected with 
this memoir have been almost entirely collected from fish 
caught in the vicinity of Piel. The author is indebted to 
Mr. R. Newsham, Jun., the Laboratory Assistant at Piel, 
* A, Scott. Trans. Nat. Hist. Soc., Glasgow, vol. iil., part 3, p. 266. 1892. 
