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Show 92 PISCES. contribute to this effect. The limbs being thus of but little use, are greatly reduced ; the parts analogous to the bones of the arms and legs are extremely short, or even completely concealed ; rays, more or less numerous, which support membranous fins, form a rude representation of the fingers and toes. The fins which correspond to the anterior extremities are termed pectorals, and those which answer to the posterior ones, ventrals. Other rays attached to particular bones placed on or between the extremities of the spinous apophyses support vertical fins on the back, under the tail, and at its ex· tremity, which, by being raised or lowered, increase or diminish the surface which strikes against the water. The superior fins are called dorsal, the inferior anal, and that at the end of the tail caudal. The rays are of two kinds; some of them consist of a single bony piece, usua11y hard and pointed, sometimes flexible and elastic, divided longitudinally-these are called spinous rays; others are composed of a great num· her of sma11 articulations, and are generally divided into branches at their extremity-they are the soft, articulated, or branched rays. There is as much variety among Fishes, with respect to the number of limbs, as among Reptiles. Most generally there are four; some have but two, and in others they are totally want· ing. The bone which is analogous to the scapula, is sometimes held among the muscles as in the higher animals, and at others is attached to the spine, but most commonly it is sus· pended on the cranium. The pelvis rarely adheres to the spine, and very frequently, instead of being behind the abdomen, is before it, and connected with the humeral apparatus. The vertebrre of Fishes are united by concave surfaces fi11ed with cartilage which most generally communicate by a canal excavated in the axis of the vertebrm. In most of them they have long spinous processes which maintain the vertical form of the body. The ribs are frequently soldered to the transverse processes. The head varies more as to form than that of any other class, notwithstanding which it almost always consists of the same number of bones as is found in other oviparous animals. PISCES. 93 The frontal bone is composed of six pieces; the parietal of three; the occipital of five; five pieces of the sphenoides and two of each temporal bone, remain, in the composition of the cranium. Besides the usual parts of the brain which are arranged as in Reptiles one after the other, Fishes have knots or ganglions at the base of their olfactory nerves. Their nostrils are simple cavities at the end of the muzzle almost always perforated by two holes, and regularly lined by a plaited pituitary membrane. The cornea of their eye is very flat, and there is but little aqueous humour, but the crystalline !s very hard and almost globular. Their ear consists of a sac representing the vestibule, in which are suspended small bodies most commonly of a stony hardness, and of three membranous semi-circular canals, situated in the cavity of the cranium rather than in the substance of its parietes, the Chondropterigii excepted, in which they are entirely contained in them. The eustachian tube and tympanal bones are always deficient, and the Selachians alone have a fenestra ovalis which is level with the head. The sense of taste in Fishes can have but little energy, as a great portion of the tongue is osseous, and frequently furnished with teeth and other hard parts. The body in most of them is covered with scales, and none possess organs of prehension ; the fleshy cirri of some may supply the imperfection of the other organs of touch. In the greater number, the intermaxillary bone forms the edge of the upper jaw, having behind it the maxi11ary, termed the labial bone. A palatine arch, composed of the palatine bones, of the two pterygoid processes, the zygomatic process, the tympanum and squamous portion, forms, as in Birds and Serpents, a sort of anterior jaw, and furnishes, behind, an articulation for the lower jaw, which generally has two bones on each side; the number of these pieces, however, is reduced in the Chondropterygii. Teeth are found in their intermaxillary, maxillary, lower jaw, vomer, bones of the palate, on the tongue, on the arches \ |
