106 



Introduction. - 



-FISHES.- 



-Stbucture. 



posterior, designing thereby to render the term more 

 applicable as a distinctive character to his order of 

 Acanthopti'fcs, whoso ventrals have a spinous ray in 

 front, and need tlie firm support of tlie coracoids wlien 

 used as a defensive weapon. Only in a very few 

 instances are the pubic bones united directly to the 

 spinal column as the hind legs are in quadrupeds ; they 

 are almost always merely suspended in the soft parts. 

 Tlie term apodal, applied to fishes, indicates that the 

 ventrals are absent ; and ahrachial, in like manner, 

 denotes the non-existence of pectorals. 



Lateral liije. — Most fishes have pores in the skin 

 from which a lubricating fluid is poured out, in some 

 very copiously, as in the glutinous hag, for which it 

 suffices to convert the surrounding water into a jelly. 

 These pores are rarely absent from a row of dermal 

 bones that run under the eye, and are tlierefore 

 named suh-orhikds ; and they may generally be traced 

 over the temples, in a chain of ossicles called sur- 

 teniporals, and onwards by the suprascapula to the 

 tjitle of the fish, which they traverse towards the tail, 

 following the line of decussation of the two great lateral 

 muscular masses. In this position the porous row 

 acquires the name of the " lateral line," and it is a 

 character of which special mention is generally made 

 in the description of the external aspect of a fish. The 

 scales that it traverses most commonly differ from the 

 others in size, figure, or in being perforated, or tubular, 

 or notched, and they form the points of meeting 

 between the oblique rows of dorsal and ventral scales. 

 In many fishes which have no other scales, the ducts 

 of the lateral line are protected by cartilaginous or 

 osteoid cases. Sometimes the lateral line runs only a 

 short way behind (he pectoral, sometimes it goes to the 

 base of the caudal rays, sometimes onward to their 

 tips ; or it may be interrupted, and recommence again 

 on a lower level ; or, finally, it may be altogether 

 imperceptible or absent, as in most sharks ; or there 

 may be two, three, or many lateral hues, as in the 

 Jlullets and Chiri. 



\VATi;R-TumiS. — Agassiz has discovered that in 

 many fishes there exist external openings which have 

 hitherto been mistaken for muoiferous pores, but whose 

 function is to admit water into the circulating system 

 through a series of tubes that end in the blood-vessels 

 near the heart. They are easily seen in the head of 

 the common shad, but none have been detected in the 

 Selachians; and they are generally few in fishes that 

 live habitually in shallow waters. He considers them 

 to be safety-tubes for balancing the system on sudden 

 alterations of pressure. An analogous system exists 

 in Molluscs, by which water can be admitted into the 

 heart from the exterior.* 



Endo-skei.eton. — The very numerous groups of 

 fishes cannot be described or recognized without a 

 more frequent reference to the parts of the internal 

 skeleton than is requisite in characterizing the fewer 

 divisions of the Mammals or Birds. But we are 

 restrained in this work from lengthened anatomical 

 descriptions, and must confine our brief notices of the 

 bones to those pieces whose names enter into the 

 definitions of the family groups of fishes. 

 * Uulliiers, rrucucd. Hoy. Soc. x. No. 37, \i. 193. Deo. 1850. 



In the skull of an osseous fish five ridges are some- 

 times very conspicuous. A mesial one, chiefly deve- 

 loped on the super-occipital, wliich is the hindmost 

 bone on the top of the skull, and ij conspicuous it. 

 figure C of Plate 17. The distal point of this ridge is 

 frequently called the " occipital spine." This mesial 

 ridge is often prolonged forwards on the mid-Jrontal 

 between the orbits. Next follow the pair of intermediate 

 ridges, one on each side developed on the par-occipi- 

 tal, exteriorly to the super-occipital, and running for- 

 wards over \\\Q parietal and mid-frontal near the upper 

 edge of the orbit. To the distal point of the interme- 

 diate ridge one limb of the chevron-shaped supra- 

 scapula is attached. Still more exterior or lower down 

 on the side of the head run the external ridges, also a 

 pair, one on each side. To the distal end of tliis ridge, 

 which is a process of the mastoid bone, the lower limb 

 of the suprascapula is attached ; the fore part of the 

 ridge reaches to the hinder margin of the orbit on the 

 p)ost- frontal. Fishes with round heads like the loaches 

 want these five ridges ; some, as the carp, have merely 

 the occipital spine. In the Chstodons the mesial ridge 

 is greatly elevated, and in the Sclerogenids the whole 

 five ridges are more or less conspicuous. 



Tiie upper profile of the face or snout is supported 

 by the nasal hone; and on each side of it are the 

 nostrils, opening by one or two small orifices into the 

 pituitary sacs, but not penetrating to the cavity of the 

 mouth. Under the nasal bone are the jaws, variously 

 formed in various groups of fishes, but retaining their 

 relative positions and connections pretty exactly. The 

 premaxillaTics, a pair of bones applied to each other 

 on the mesial line, form the middle part of the upper 

 jaw, and in a considerable number of osseous fishes 

 border the entire upper half of the mouth. Articulated 

 to the palatine, behind the premaxillary, lies the 

 maxillary, whose lower end in many fishes comes for- 

 ward to the corner of the mouth, and enters into the 

 composition of its upper border. When the jaws are 

 protractile, each premaxillary sends out a long pedicel, 

 which glides backwards and forwards by the side of the 

 nasal on a tubercle of the maxillar}'. The mandible, 

 composed of two limbs, each consisting of two or more 

 pieces, makes up the lower jaw. Underlying the nasal, 

 within the roof of the mouth on the mesial line, is the 

 vomer ; and on each side of it is one of the pair of 

 palatine bones. In the back part of the roof of the 

 mouth, over the gullet, is the siib-occipital, which is 

 confluent with the sphenoid that articulates anteriorly 

 and above with the vomer. All the bones here named 

 as belonging to the orifice of the mouth or its roof are 

 dentiferous in one species of fish or another. Interiorly 

 between the limbs of the mandible there is often a 

 lingual or tongue-bone, frequently bearing tectli, and 

 opposed to the vomer. There are also in almost ail 

 cases teeth on the branchial arches or on parts of the 

 hyoid bones connected with them. 



The Gills and Hyoid Appakatus. — Descending 

 from the mastoid bones of each side a series of pieces 

 meet at the tongue, and form an elastic inverted chevron 

 like the letter V. Pamning backwards in the lower 

 angle of the V, and on the mesial line from the tongue, 

 between the limbs or cornua of this arch, there is a 



