Intkoduction.- 



-FISHES.- 



-Structure. 



107 



narrow isthmus of ossicles, which are froqueiitly studded 

 with hair-lilce teeth; and, rising in bacliward curves 

 from tlie isthmus, there is a series of successive pairs 

 of branchial arches, whose upper ends are connected to 

 the sub-occipital by membrane. Each arch consists of 

 several pieces with joints, rendering it pliable and 

 clastic ; and each has, a little above the middle of its 

 limb, a larger joint, which enables it to fold up on itself 

 when necessary'. On the interior edge of each arch 

 there are two rows of knobs, cones, plates, or lancet- 

 shaped processes beset with fine teeth ; these are 

 named ral-ers, and act as sieves in straining the water, 

 and preventing gross substances from passing between 

 tlie arches. To the extero-posterior side of each arch 

 are attached the gills, whose general aspect needs uo 

 description, but which present considerable modifica- 

 tions of form and arrangement in particular groups of 

 fishes. Generally there are fonr gill-bearing arches, 

 but sometimes the gills are fewer. In the embryo fish 

 six arches may be perceived, and even seven are 

 indicated, but the first is atrophied before the sixth is 

 developed, and only five are matured. The last arch 

 does not in general carry gills, but is modified into a 

 pair of pliaryngcal jaws, being dentifcroiis on its sides, 

 having two bony denial plates or groups of plates 

 below named the infirior pharyt^geals, and very fre- 

 quently similar superior plates lining the sub-occipital 

 and called sujyerior 2}haryngeuls. The union of the 

 inferior phar\'ngeals into a single bone is an essential 

 character of the group of Piiaryngognatiis. 



In cartilaginous fishes the jaws and parts of the skull 

 are so different as to need special descriptions. The 

 gills of these Selachians, though adherent to the sides 

 cf gill-chambers, each of which opens externally, have 

 essentially the same ultimate structure and the same 

 function as the gills of fishes proper, which is to submit 

 the blood extensively to the action of the oxygen 

 contained in the waters that fishes inhabit. 



In osseous fishes the tips of the gills are free, and there 

 is only one external opening on each side, by which all 

 the water that bathes the gills flows out. This opening is 

 regulated by a valve called the operculum or gillcorer, 

 which consists of four pieces — viz., i\\a pre- oj>ercuhtm, 

 a chevron-shaped or crescentic bone, bounding the 

 cheek posteriorly ; the operculum, often quadrangular, 

 and filling the space between the pre-operculum and 

 upper part of the gill-opening; the suh-opercuhim, lying 

 as its name denotes under the operculum, and bordering 

 the lower part of the gill-opening ; and, fourthly, of the 

 inter-operculum, which lies before the sub-operculum, 

 and runs forward under the lower limb of the pre-oper- 

 culum towards the mandible. Between the sub-oper- 

 culum and inter-operculum of the one side and those 

 of the other a membrane named the hranchiostegous 

 stretches across the throat, and fills the interval between 

 the limbs of the mandible. It is distended on rib-like 

 bones called hranchiostcfials, which are articulated to 

 the horns of the hyoid bone. Care is generally taken 

 to note the number of the branchiostogals in the 

 description of a fish, and in some groups the number is 

 very constant, furnishing in that case a good character. 

 When the gill-opening is large, and runs forwards to 

 the root of the tongue, the hranchiostegous membranes 



are narrow and are completely separated from eacli 

 other. On the contrary, the membranes are continued 

 into one another when the gill-opening is small and 

 high up. 



The Heaut.— In fishes all the blood is propelled 

 through the gills by the heart, which, lying behind 

 them, is protected by the coraeoid arch. This central 

 organ of the circulation consists of a single series of 

 chambers — viz., in the osseous fislics, of the raioim 

 sinus, the auricle, the ventricle, and the arlcrial bulb 

 or stem. There are generally two valves at tl.e auriculo- 

 ventricular opening; but the Rays have three valves 

 there, the Sun-fish four, and in the Sharks there is one 

 large valve like a jiarachute with several marginal 

 attachments. The numbers and arrangement of the 

 valves in the arterial stem serve as a primary character 

 of the Ganoid fishes ; and in the Sharks and Kays also 

 the valves are numerous. 



SwiM-BLADDER. — Tlie swim-bladdcr, or air-bladder, 

 is an organ peculiar to fishes, and lies in the belly near 

 the spinal column. In some fishes it is closed com- 

 pletely, and then the included air is found to consist 

 chiefly of oxygen ; in other fishes it opens through 

 a tube directly into the stomach or oesophagus by a 

 valvular orifice, and then it is found to be distended 

 with azote and a small proportion of oxygen; but in no 

 case have its contents been found to be atmospheric 

 air. In some groups of fishes the air-bladder is con- 

 nected with the acoustic organs by chains of small 

 bones. 



Hearing. — In the class of fishes the organs of hear- 

 ing are wholly within the head and cannot be disco- 

 vered except by dissection, the Plagiostomes alone 

 having a fine tube leading to an external orifice from 

 the interior labyrintli. This external opening is not 

 easily detected, and it is not surprising that the sense 

 of hearing has been denied to fishes by some authors ; 

 but many facts may be cited both from ancient and 

 modern writers, showing that when kept in stews fishes 

 will come to the call of those that feed them, or may be 

 assembled at the sound of a whistle or bell. It is 

 probable that the senses are less acute in fishes than 

 in animals higher in the vertebral scale ; but as they 

 are furnished with nerves of touch, taste, sight, and 

 smell, as well as of hearing, there is no reason to s\ip- 

 pose that they are not endowed with all tlie five senses 

 in greater or less perfection ; and as certain large nerves 

 are in a few species supplied to galvanic or electric 

 organs, we have an instance of a nervous function 

 peculiar to this class. A notice of this galvanic power 

 wiU be more appropriate when we have to speak of the 

 groups in which it is displayed. 



With respect to intelligence, fishes in stews have 

 been known to recognize those that feed them ; and 

 we have seen one kept in a glass tank that, though 

 very shy of strangers, evidently distinguished the lady 

 who supplied it with food. The gregarious fishes that 

 move about in large sculls cannot be expected to evince 

 much individual attachment ; but some that live in 

 pairs are known to have refused to eat, and to pino 

 away wdien deprived of their companion. Many fishes 

 make elaborate nests for their eggs, some instances of 

 which will bo alluded to hereafter. 



