OCR Text |
Show 1899.] PARASITIC COPEPODA ON FISHES. 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 N e w 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 Brachiella thynni, and was mentioned by Aristotle, Pliny, and Eondeletius. Strom, a long time ago, mistook the tail for the head of a Caligus, and the egg-tubes for antennae. De Blainville thought the eye of a Sprat was the head of Lernceenicus sprattce; and more recently M . P. Van Beneden (as Carl Vogt has pointed out) has described the Leposphile 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 Lernaeas often have the body (which is soft, and generally of a reddish colour, from the haemic fluid inside, and therefore not bad food for small fish) covered with a growth of algae 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 Lernceolophus, and not until examined with a lens was their true nature detected. Tbe body-portion of Sphyrion is often entirely hidden with this secondary parasitic growth, and as they themselves are furnished with hard processes, like bunches of calcareous algae, 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 Caligidce found on the exterior 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 never have I seen so great a disproportion 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 may be divided into those which are blood-suckers and those which are mucus-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 young attached condition of some of the Caligidae has been 29* |