STRUCTURE OF FISI1ES. 15 



circular; there are also, as in higher and lower animals, larger white corpuscles. The greatest 

 quantity of blood is found in the Tunny, which, according to Dr. Davy, is a warm-blooded 

 animal, with a temperature as high as that of most mammals. The lowest type of fish 



the Lancelet has no true heart, but a blood-vessel which pulsates very slowly, there being 



but one beat in a minute, while in most fishes the number of beats in a minute varies from 

 twenty to twenty-four. The heart is relatively small in all fishes, and even in Sharks and 

 Rays is only about one-thousandth part of the weight of the body, which is about one-half of its 

 relative size in reptiles. This organ in. fishes consists chiefly of two parts, which correspond to the 

 pulmonary side only of the heart in man, and are named the auricle and ventricle. It is usually placed 

 in the throat, in a cavity partitioned off by a fixed tendinous diaphragm. The auricle, which is thin and 

 relatively large, receives its blood from a sinus, formed by the union of the veins which bring the 

 blood from the body. Each aperture of the auricle is guarded with strong valves. The ventricle is 

 small and muscular; its form is pyramidal in the Ganoid fishes, lozenge-shaped in the Pike, oval in the 

 genus Lophius. It opens by means of valves into an enlarged muscular portion of the branchial artery, 

 which is called the bulbus arteriosus ; this contracts like the ventricle, and assists in forcing the 

 blood into the gills. In the interior of this organ there is an elaborate system of valves, which are 

 arranged in two rows in most Sharks, and form three, or even six, rows in Ganoids. After the blood 

 has passed through the gills the vessels unite and form an artery, which extends under the vertebra 

 along the length of the body. 



The modifications of the gills are sufficiently important to give names to some of the great group 

 of fishes, such as Marsipobranchii, Lophobranchii, and Elasmobranchii. In the Hag, which is an 

 example of the Marsupial type of gill, there are six little branchial sacs on each side ; these are 

 produced into short tubes on both sides, and these tubes are prolonged into a longitudinal canal, which 

 extends backward, and carries the stream of water away from the gills on each side, terminating on 

 the ventral surface on each side of a third larger opening, which admits water in the same way into 

 the branchial sacs. In the Lophobranchii, which compris3 the Pipe-fishes, the gills, instead of having 

 the comb-like form usual among fishes, form a double series of nearly circular tufts. In the Elasmo- 

 branchii the gills are arranged side by side, so as to suggest the idea of plates with openings 

 between them, which are usually long slits, as may be seen in the Sharks and Rays. The branchial 

 chamber is largest in those fishes in which the outlet from it is small. In some of the Eels these 

 outlets approximate close together on the under side of the head ; in the Sturgeons and Ganoids 

 there is a canal leading from the fore part of each side of the branchial chamber to the top of the head. 

 These canals are called spiracles. In all the osseous fishes there is only one visible outlet to the gills 

 on each side. Each leaflet of the gills usually consists of a pair of processes, but in some osseous 

 fishes some of the branchial arches support only one series of these leaflets. 



Many genera, like Zeus and Polypterus, have on each side three double gills and one uuiserial. 

 In the genera Lophius and Diodon there are three bi-serial gills ; in Lepidosiren there are bi-serial gills 

 and one uniserial. The number of plates on a single leaflet may range from as few as fifty-five in the 

 Gudgeon to sixteen hundred in the Sturgeon. It is interesting to remark that in the embryonic 

 osseous fish the five interspaces between the hyoid arch and the five branchial arches are exposed on 

 the sides of the head, and that subsequently the branchiostegal appendages are developed, and a 

 single branchial outlet results from the formation and backward growth of the operculuin. Owen 

 remarks that the mechanism of breathing in fishes differs from that of swallowing only in the streams 

 of water not entering the gullet, and being diverted to the branchial slits on each side of the pharynx. 

 The bones which cover the gills are collectively known as the opercuhim. This organ is connected 

 with the skull by means of the hyo-mandibular bone, which also supports the jaws. The principal 

 bone is named the operculum, below which is the sub-operculum, and below this the inter-operculum. 

 In front of these is the pre-operculum. 



It is necessary to complete our knowledge of fishes to carefully examine the characters of their fins. 

 In the majority of fishes there are five kinds of fins, which are named pectoral, ventral, anal, dorsal, and 

 caudal (see p. 3). The pectoral and ventral correspond to the arms and legs of higher animals, 

 and when they exist they are always in pairs. The other fins are single or unpaired, and are entirely 

 unrepresented in the skeletons of higher Vertebrates. The pectoral fins are almost always present, 



