ICHTHYOLOGY. 



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the external opening* are two in number, but there are 

 six lateral bags on each side, placed perpendicularly, 

 to which there are six tubes from each of the openings, 

 and close to the left external opening, there is one which 

 passe; directly into the oesophagus. ( Phil Trans. 1 8 J 5, 

 Part 1 1. p. 236.) 



The characters furnished by the gills are of the first 

 importance in arranging the species. They are easily 

 examined, and the distinctions are obvious. But by 

 some strange conceit, ichthyologists seldom look into 

 the gills, or point out to us the peculiarities which 

 they exhibit. Indeed the characters furnished by the 

 organs of respiration should be regarded as occupying 

 the highest rank. Without the aid of any other cha- 

 racters, fishes might be classified with ease, and even 

 species might be determined with certainty. 



SECT. VII. Orgnm of Circulation. 



THE organs of circulation are not so obvious as those 

 which we have been considering, and are seldom at- 

 tended to by the mere ichthyologist. Without entering 

 into the minute details of comparative anatomy, we 

 trust the following observations may not prove uninte- 

 resting to the general reader. The heart of fishes is 

 situated in the forepart of the body, in a cavity between 

 the gilt* and a little behind. The pericardium or mem- 

 brane which line* thin cavity, is similar to the covering 

 of the cavity of the abdomen, and like it, is often spot- 

 ted or silvery. In the skate, Dr Monro found the bot- 

 tom of the pericardium lengthened into the shape of a 

 fennel, which divide* into two brandies. These are 

 tied closely to the lower part of the oesophagus, and 

 open into the cavity of the abdomen. Into this cavity 

 there is secreted a liquor, afterwards to be taken notice 

 of. The heart itself is small in proportion to the body 

 of the animal, and varies greatly in figure in the diffe- 

 rent specie*. It is quadrilateral in some, and semi- 

 circular in other*. It consists, a* we have already men. 

 >in*Mnl, of a single auricle and a single ventricle, cor- 

 responding to the right side of the heart of warm. 

 hloaded animals. The auricle is in general larger than 

 the ventricle, and of a thinner texture in its coat*. It 

 neceirea the blood from the body, and transmits it to 

 the ventricle. Thi* last division of the heart has wall* 

 of considerable thicknea*. It send* forth an artery, 

 which, at ha separation from the heart, forms a bulb 



> shape according to the specie*. Thi* ar- 

 videa and proceed* directly to the gill*, over 

 leave* it i* spread in the most minute ramioca. 



The Mood, hi paeaing through the gill* of fishe*, un- 

 dergo** eimilar change* a* the blood of quadruped* in 

 passing through their lung*. It is likewise now well 

 aetahliahxl, that the atmospheric air contained in the 

 water furnishes to the Mood those material* which are 

 Meeetary for its purification, and a continuance of the 

 life of the animal 



The water tor thi* purpose i* taken in at the mouth, 

 eat to the cilia, where, after being in a great mea- 

 deprived eftbe oxygen of it* armoaphenc air, the 

 ia ejected through the gill opening. 



" In the lamprey," according to Home, " the water 

 i* received by the lateral openings of the animal into 

 the beg* which perform the office of cilia, and paate* 

 out by the Mme opening ; the form of the cavities be- 

 ing fitted to allow the water to go in at one side, pas* 

 Mmd the projecting parts, and out at the other. A 

 pert of the water escape* into the middle tube, and 



Structure 



and 



Functions 

 of Kislies. 



from thence either passes into the other bags, or oat at 

 the upper end into the oesophagus. There is a com- 

 mon opinion that the water is thrown out of the nos- 

 tril: this, however, is unfounded, as the nostril has 



no communication with the mouth." " In the myxine, 

 the elasticity of the two tubes, and the bags into which 

 they open, admits of the water being received ; and the 

 pressure produced by the action of the external mus- 

 cles forces it into the oesophagus, from whence it is 

 thrown out by the opening at the lower end of that 

 tube." 



If the ejection of the water from the gills of a fish 

 be prevented, by the gill cover being tied down with a 

 string, it soon expires in convulsions. Similar fatal con- 

 sequences follow, when fish are placed in water previ- 

 ously deprived of its atmospheric air by boiling or 

 freezing : and when fish are kept in a small pond, whose 

 surface is frozen over, and where the water in that case 

 cannot obtain a fresh supply of air, they speedily pe- 

 rish. If a small opening be made in the ice, before it 

 be too late, the fishes will come near it for a fresh sup- 

 ply. In this manner, fishes are frequently taken du- 

 ring winter in ponds and lakes. 



The extent of surface presented by the gills of a fish, 

 to enable the blood to come in contact with the air in 

 the water, is much greater than one would, without 

 attentive consideration, be led to suppose. Dr Mount 

 calculated, that the whole gills of a large skate present- 

 ed a surface equal to 2250 square inches, or equal to 

 the whole external surface of the human body. 



The process of respiration for the supply of the gills 

 is carried on even during sleep. The number of respira- 

 tions in a minute is seldom above thirty, or below twenty. 

 In the same individual it is liable to considerable varia- 

 tion, depending on the will of the animal. 



The blood, after being renovated in the gills, is re- 

 absorbed by a multitude of minute vessels, which unite 

 together ; but, instead of returning the blood to the 

 heart again, to be afterwards distributed through the 

 body, this aorta exercises that function, and descends 

 along the inferior side of the spine, in a canal fitted 

 for its reception, giving off arteries, during its course, 

 to the adjacent part*. The blood is absorbed again by 

 veins, which have extremely thin coats. These are 

 much larger in their course than in their termination ; 

 and besides form, in different parts of their course, con- 

 siderable receptacles for blood. 



Any injury received by the gills of fishes is attended 

 with much pain, and a considerable effusion of blood. 

 Some fishermen seem to be well aware of this last circum- 

 stance, and cut the gill* with a knife as soon as the fish 

 is taken. A copious bleeding takes place ; and they 

 find that a fish so killed will Keep much longer in a 

 fresh state, than one on whom this operation of bleed- 

 ing ha* not been performed. 



SICT. VIII. Organs of Nourithment. 



IN attending to the organs of nutrition, it will be ne- Organs of 

 cessary to consider the structure of the mouth, and af- nourish- 

 terwards the gullet, stomach, and intestines. 



The ttiuulh of fishes exhibits many remarkable dif- Mouth, 

 ferences, according to the species, in regard to position, 

 figure, and size. In general, it is situated at the ex- 

 tremity of the head, and is then said to be terminal. 

 In some species, and even genera, it is placed beneath 

 a snout, or on the under side of the head. Wherever 

 situated, it is always transverse with respect to the bo- 

 dy, unless in the genus Pleuronectes, in which it oc' 



