654 ICHTHYOLOGY [XUTBITION dead to external impression at the time of copulation, and, as it were, so automatically engaged, that they have been taken by the hand together out of the water. ORGANS OF NUTRITION AND DIGESTION. Fishes are for the most part either exclusively carnivorous or herbivorous, but not a few feed on vegetable as well as animal substances, or on mud containing alimentary matter in a living or decomposing state. Generally they are very voracious, especially the carnivorous kinds, and the rule of "eat or be eaten" applies to them with unusual force. They are almost constantly engaged in the pursuit and capture of their prey, the degree of their power in these respects depending on the dimensions of the mouth and gullet and the strength of the teeth and jaws. If the teeth are sharp and hooked, they are capable of securing the most slender and agile animals ; if with teeth of this kind are combined a wide gullet and distensible stomach, the fish is able to overpower and swallow others larger than itself ; if the teeth are broad, strong molars, they are able to crush the hardest alimentary substances; if they are feeble, they are only serviceable in procuring some small or inert and unresisting prey. Teeth may be wanting altogether. Whatever the prey, in the majority of cases it is swallowed whole ; but some of the most voracious fishes, like some sharks and Characinidce, are provided with cutting teeth, which enable them to tear their prey to pieces if too large to be swallowed whole. Auxiliary organs, similar to the claws of some carnivorous mammals and birds, for the purpose of seizing and overpowering their prey before it is torn by the teeth, are not found in this class ; but in a few fishes the jaws themselves are modified for that purpose. In the sword-fishes the bones of the upper jaw form a long dagger-shaped weapon, with which they not only attack large animals, but also frequently kill fishes on which they feed. The saw-fishes are armed with a similar but still more complex weapon, the saw, which is armed on each side with large teeth implanted in deep sockets, specially adapted for killing and tearing the prey before it is seized and masticated by the small teeth within the mouth. Fishes show but little choice in the selection of their food, and some devour their own offspring indiscriminately with other fishes. Their digestive powers are strong and rapid, but are affected in some degree by the temperature, which, when it sinks below a certain point, lowers the vital powers of these cold-blooded animals. On the whole, marine fishes are more voracious than those inhabiting fresh waters ; and, whilst the latter may survive total abstinence from food for weeks or months, the marine species succumb to hunger within a few days. The organs of nutrition, manducation, and deglutition are lodged in two large cavities an anterior (the mouth or buccal cavity), and a posterior (the abdominal cavity). In the former the alimentary organs are associated with those fulfilling the respiratory functions, the transmission of food to the stomach and of water to the gills being performed by similar acts of deglutition. The abdominal cavity com mences immediately behind the head, so, however, that an extremely short thoracic cavity for the heart is partitioned off in front. Besides the alimentary organs it contains also those of the urogenital system and the air-bladder. The abdominal cavity is generally situated in the trunk only, but in numerous fishes it extends into the tail, being con- tinusd for some distance along each side of the haemal apophyses. In numerous fishes the abdominal cavity opens outwards by one or two openings. A single porus abdominalis in front of the vent is found in Lepidoslren and some stur geons ; a paired one, opening on each side of the vent, in Ce- ratodus, some species of sturgeon, Lepidosteus, Polypterus, Amia, and all Chondropterygians. As in these fishea semen and ova are discharged by their proper ducts, the abdominal openings may serve for the expulsion of semen, and of those ova only which, having lost their way to the abdominal aperture of the oviduct, would be retained in the abdominal cavity. In those Teleosteans which lack an oviduct a single porus genitalis opens behind the vent. Mouth. The mouth of fishes shows extreme variation with regard to form, size, and position. Generally opening in front, it may be turned upwards, or it may lie at the lower side of the snout, as in most Chondropterygians, sturgeons, and some Teleosteans. In most fishes the jaws are covered by the skin, which, before passing over the jaws, is often folded, forming more or less fleshy lips. In the sharks the skin retains its external character even within the teeth, but in other fishes it changes into a mucous membrane. A tongue may exist as a more or less free and short projec tion, formed by the glossohyal and a soft covering, or may be entirely absent. Salivary glands and a velum palati are absent in fishes. Teeth. With regard to the dentition, the class of fishes offers an amount of variation such as is not found in any of the other classes of vertebrates. As the teeth form one of the most important elements in the classification of fishes, their special arrangement and form will be referred to in the account of the various families and genera. Whilst not a few fishes are entirely edentulous, in others most of the bones of the buccal cavity, or some of them, may be toothed, as the bones of the jaws, the palatines, pterygoids, vomers, basi-sphenoid, glossohyal, branchial arches, upper and lower pharyngeals. In others teeth may be found fixed in some portion of the buccal membrane without being supported by underlying bone or cartilage ; or the teeth have been developed in membrane overlying one of the dentigerous bones mentioned, without having become anchylosed to the bone. When the tooth is fixed to the bone the attachment has generally been effected by the ossification of the bone of the tooth, but in some fishes a process of the bone projects into the cavity of the tooth ; in others the teeth are implanted in alveoli. In these, again, frequently a process of bone rises from the bottom, on which the tooth rests. Many of the class, especially predatory fishes, with long, lancet-shaped teeth, have all or some of these capable of being bent towards the interior of the mouth. Such " hinged " teeth resume at once the upright position when pressure is removed from them. They are, however, depressible in one direction only, thus offering no obstacle to the ingress, while they oppose the egress of prey. Mr C. S. Tomes has shown that the means by which this mechanism is worked are different in different fishes ; for, whilst in the Pediculati and Gadoids (hake) the elasticity resides solely in the tissue of the hinge (the tooth being as resilient as ever after everything else is severed), in the pike the hinge is not in the least endowed with elasticity, but the bundles of fibres proceeding from the interior of the dentine cap are exceedingly elastic. The teeth may be, and generally are, very different as regards size or form in the different parts of the mouth ; they may be also different according to the age or sex of the fish (Raici). The teeth may be few in number and iso lated, or placed in a single, double, or triple series, distant from one another or closely set ; they may form narrow or broad bands, or patches of various forms. As regards form, they may be cylindrical or conical, pointed, straight, or curved, with or without an angular bend near their base ; some are compressed laterally or from the front backwards (the latter may be triangular in shape, or truncated at the top like the incisors of mammals) ; they may have one apex
(cusp) only, or be bi- or tri-lobate (bi- or tri-cuspid), or may
