OKDERS OF FISHES. 219 



and known as the " interspinous " bones (Fig. 101). The me- 

 dian fins are variable in number, but when fully developed 

 they consist of one or two fins on the back (the dorsal fins), 

 one or two on the ventral surface (the anal fins), and one 

 clothing the posterior extremity of the body (the caudal fin, 

 or tail, Fig. 102). In all the Teleostei, the caudal fin has the 

 shape called " homocercal " that is to say, it consists of two 

 equal lobes and the vertebral column is not prolonged into 

 the upper lobe (Fig. 103, a}. 



5. The heart consists of two cavities, an auricle and a ven- 

 tricle, but the bulbus arteriosus is not rhythmically contractile, 

 and is separated from the ventricle by only a single pair of 

 valves. 



6. The respiratory organs are in the form of free, comb- 

 like, or tufted gills, enclosed in two cavities placed on the 

 sides of the neck. Each of these branchial chambers opens 

 externally by a single aperture, the " gill-slit," which is pro- 

 tected by a chain of bones, forming the " gill-cover," and by 

 a membrane supported by bony rays. Internally the branchial 

 chambers communicate with the throat by a series of clefts or 

 fissures, and the water required in respiration is taken in at 

 the mouth by a process analogous to swallowing. 



7. The nasal sacs never communicate behind with the 

 throat (pharynx). 



TABULAR VIEW OF THE MAIN DIVISIONS OF THE TELEOSTEI. 



SUB-ORDER I. MALACOPTERI. Usually a complete series of fins, supported 

 by rays, all of which are soft, or many-jointed (with the occasional exception 

 of the first rays in the dorsal and pectoral fins). A swim-bladder is always 

 present, and is always connected with the gullet by a duct. The skin is rarely 

 naked, and is mostly furnished with cycloid scales, but sometimes ganoid 

 scales are present. 



Among the more important families in this sub-order are the Eels (Mu- 

 rcenidce), Herrings (Clupeidce], Pikes (Esoddce), Carps (Cyprinidce), Salmon 

 and Trout (Salmonidai), and Sheat-fishes (Siluridce). 



SUB-ORDER II. ANACANTHINI. Fins entirely supported by soft rays, and 

 never by spinous rays. Ventral fins either wanting, or placed under the 

 throat, beneath or in advance of the pectorals. 



The two leading families in this sub-order are the Cod, Ling, and Haddock 

 family (Gadidce), and the Flat-fishes (Pleuronectidce\ comprising the Sole, 

 Turbot, Flounder, and others. 



SUB-ORDER III. ACANTHOPTERI. Fins with- one or more of the first rays in 

 the form of undivided, inflexible, spinous rays. Scales mostly ctenoid. Swim- 

 bladder without a duct. 



The leading families in this order are the Wrasses ( Cyclo-labridce\ the 

 Perches (Percidce), the Mackerels (Scomberidce), the Mullets (Mugilidce], and 

 the Gobies (Gobiidce). 



