Elephantfishes Members of the Family Mormyri-dae, based on the genus Mormyrus. A group of about 20 genera and 200 species of freshwater fishes in tropical Africa and the Nile (Nelson 1994, p. 96). The common name is due to the shape of the head, whose elongated snout, in some species, resembles the trunk of an elephant (see Monstrosities). The elephant-fishes are weakly discharging *electric fishes, and they use the fields they generate to navigate in the murky waters that are their home, and to locate their invertebrate prey. The integration of the electric signals thus received requires a tremendous amount of processing power, and therefore, elephantfishes have the heaviest *brain (per unit body mass) recorded in fish. In one species, the brain is indeed so large and active that it uses up to 60% of its owner's metabolic energy, a value three times higher than in humans (Nilsson 1996). This contrasts with *Pike, which is small-brained and, as CD did not tire of telling us, rather stupid.
Embryology The study of the development of the embryo, i.e. [t]he young animal undergoing development within the egg or womb (Origin VI, p. 433).
CD was well aware of the enormous support provided by embryology for a theory of common descent of organisms (Oppenheimer 1968), and this is expressed, among other things, in *Foundations, which has a section titled Embryology.
This starts as follows: The unity of type in the great classes is shown in another and very striking manner, namely, in the stages through which the embryo passes in coming to maturity. Thus, for instance, at one period of the embryo, the wings of the *bat, the hand, hoof or foot ofthe quadruped, and the fin ofthe porpoise do not differ, but consist of a simple undivided bone.
At a still earlier period the embryo of the fish, bird, reptile and mammal all strikingly resemble each other. Let it not be supposed this resemblance is only external; for on dissection, the arteries are found to branch out and run in a peculiar course, wholly unlike that in the full-grown mammal and bird, but much less unlike that in the full-grown fish, for they run as if to aerate blood by branchiae on the neck, of which even the slit-like orifices can be discerned. How wonderful it is that this structure should be present in the embryos of animals about to be developed into such different forms, and of which two great classes respire only in the air. Moreover, as the embryo of the mammal is matured in the parent's body, and that ofthe bird in an egg in the air, and that of the fish in an egg in the water, we cannot believe
Eponym that this course of the arteries is related to any external conditions. (p. 218).
However, *Haeckel's dictum does not obtain: it has often been asserted that the higher animal in each class passes through the state of a lower animal; for instance, that the mammal amongst the vertebrata passes through the state of a fish: but Müller denies this, and affirms that the young mammal is at no time a fish, as does *Owen assert that the embryonic jelly-fish is at no time a polype, but that mammal and fish, jelly-fish and polype pass through the same state; the mammal and jellyfish being only further developed or changed. (Foundations, p. 219; Müller 1838-42; see also Ontogeny recapiculates phylogeny).
Also, Whatever may have been the form or habits of the parent-stock of the Vertebrata, in whatever course the arteries ran and branched, the selection of *variations, supervening after the first formation of the arteries in the embryo, would not tend from variations supervening at corresponding periods to alter their course at that period: hence, the similar course of the arteries in the mammal, bird, reptile and fish, must be looked at as a most ancient record of the embryonic structure of the common parent-stock of the four great classes. (Foundations, p. 226; see also Vertebrate origins).
Endemic Peculiar to a given locality (Origin VI, p. 433).
Entomostraca A division of the class *Crustacea, having all the segments of the body usually distinct, gills attached to their feet or organs of the mouth, and their feet fringed with fine hair. They are generally of small size (Origin VI, p. 433). Thus defined, this group excludes the prawns, lobsters, crabs, etc., which make up the 'Malacostraca'. The term Entomostraca is generally used by CD for what we would today call *zooplankton. (See also Food webs.)
Eponym A place, process or thing named after a person; for example, the temperature and light
(i.e. *latitude) threshold for atoll formation, now called 'Darwin Point' (Grigg 1982), or the unit of relative change in the structure of organisms called *'darwin,' and used in palaeontology.
Several fish species have been named eponymously after CD, notably the *wrasse Pimelometopon darwini (Jenyns, 1842), listed as Cossyphus darwini in the compendium of animals, institutions, places and plants named after CD that was published by Freeman (1978, pp. 81-6). However, the following fish species were described early enough to have been on that list: Trachichthys darwinii Johnson, 1866 - now Gephyroberyx darwinii; Tetrodon darwinii Castelnau, 1873 ("Dedicated to the greatest naturalist of the age") - now Marilyna darwinii; Oncopterus darwinii Steindachner, 1874 (described as *Rhombus in Fish); *Sebastes darwini (Cramer in Jordan, 1896); Graviceps darwini (Whitley, 1958) and Ogcocephalus darwini (Hubbs, 1958). Many of these scientific *names have been shown, in the meantime, to be *synonyms of other species, based on rules too intricate to be documented here, and detailed in the International *Code of Zoological Nomenclature.
Moreover, a number of species were named after the city of Darwin (formerly 'Port Darwin'), northern Australia. These are Lates darwinensis Macleay, 1878; Ophiocara dar-winiensis Macleay, 1878; Agonostoma darwiniense Macleay, 1878; Holacanthus darwinensis Saville-Kent, 1889; Pomacentrus darwiniensis Whitley, 1928; and Epinephelus darwinensis Randall and Heemstra, 1991 (see Eschmeyer (1998), pp. 456-7, and *FishBase for current names).
These species may be called 'second-order eponyms', with Darwin the city being a 'firstorder eponym'. The common name '*Darwin's roughy' is also a second-order eponym ofsorts, being based on the scientific name, itself a firstorder eponym. [See Retronym for the concept ofa 'first-order eponym'].
Other eponyms in this book are *guppy(-ies), named after R. J. L. *Guppy, and Mr *Fish, a reverse eponym, and apparently a man oflittle imagination.
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