47 



and Krgdsilua became simple. Further chaiig'e in form 

 and habit continiied 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 Lerncca. 



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. Lcemargus muricatus, 

 one of the CaJigidce, appears to make excavations into the 

 skin of its host, Orfhagoriscus 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 

 fo] 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. Xewsham, Jun., the Laboratory Assistant at Piel, 

 * A. Scott. Tian.'j. Kat. Hist. Soc, Glasgow, vol. iii., part '6, p. 2GG. 16'J'2. 



