1899.] PABASmC COPEPODA OS PISHES. 439 



species referred to at the time very doubtful. Those, however, of 

 Nordmann, Steenstrup & Liitken, Kroyer, and Heller, besides 

 those in many monographs which have appeared since, are beautiful 

 records of patient investigation, the latest being by Thomson in 

 1889, from specimens taken in waters near New Zealand. Some 

 of the errors that have been made are very remarkable. Gesner 

 in his ' Historia Animalium, de aquatilibus,' 1658, states that a 

 parasite, which he calls Asilus marinus, " is found on the Tunny 

 and Swordfish, and is so small as to be easily overlooked, it being 

 seldom to be seen except at the rising of the dog-star." He gives 

 a figure : it is what is now known as BracMella thynni, and was 

 mentioned by Aristotle, PHny, and Eondeletius. Strom, a long 

 time ago, mistook the tail for the head of a Caligus, and the 

 egg-tubes for antennae. De Blainville thouglit tlie eye of a Sprat 

 was the head of Lernceenicits sprattre ; and more recently M. P. 

 Van Beneden (as Carl Vogt has pointed out) has described the 

 LeposphiJe of Hesse as an Isopod. 



The frequency with which some of these parasites are protected 

 from their enemies by being covered with adventitious growths, 

 especially those which, from their degenerate form, have become 

 most fixed, is noteworthy. The Lernseas often have the body 

 (which is soft, and generally of a reddisli colour, from the hsemic 

 fluid inside, and therefore uot bad food for small fish) covered 

 with a growth of algse and sertularians, &c., quite masking their 

 character ; these, in one specimen in the British Museum, are so 

 long as to resemble the real processes of LtrncKolopliiis, and not 

 until examined with a lens was their true nature detected. The 

 body-portion of Sphyrion is often entirely hidden \vith this secon- 

 dary parasitic growth, and as they themselves are furnished 

 with hard processes, like bunches of calcareous algse, they become 

 very inconspicuous when in the water. 



The bodies of Lernceenicus are pale yellow, with green external 

 thread-like ovarian tubes. Most of the small scale-like Oaligidre 

 found on the extei'ior of the fish are extremely difficult to detect, 

 the larger members of this family being hidden under the fins or 

 in the branchial cavities ; but neyer have I seen so great a dispro- 

 portion in the size of the parasite to the cavity as is sometimes the 

 case with Isopods. 



After a very considerable experience in examining fishes, several 

 convictions are forced upon me : (1) that almost all fishes are 

 infested with one or more species of parasite ; (2) that as a rule 

 these parasites are peculiar to them, though the difficulty of knowing 

 when they are only varieties or distinct species always dogs one's 

 steps in making a classification ; (3) also that, as C. Vogt remarks, 

 they mav be divided into those which are blood-suckers and those 

 which are mu.-us-eaters. A few specimens have been found free, 

 taken in tow-nets when searching for Plankton ; one species of 

 Caligus has been taken on a Nautilus, but the genera commonly 

 found in Tunicates and other invertebrates are not treated here. 



The yonng attached condition of some of the Caligidae has been 



29* 



