VERTEBRATA: FISHES. 485 



VI. The nasal sacs never communicate posteriorly with the 

 cavity of the pharynx. 



VII. The exoskeleton usually has the form of overlapping 

 horny scales of the cycloid or ctenoid character ; but it is 

 sometimes absent, sometimes composed of scattered plates of 

 true bone, sometimes ganoid, and sometimes formed of sha- 

 green-like bony spines. 



VIII. The stomach is capacious ; pyloric caeca are present ; 

 the intestine has no spiral valve; and the rectum usually opens 

 separate from and in front of the urinary and genital apertures. 

 The air-bladder may or may not be present, and may or may 

 not communicate with the gullet. The kidneys are well de- 

 veloped. The reproductive organs may be solid, and may 

 liberate their contents by rupture into the abdominal cavity; 

 but they are usually hollow organs, with ducts which open 

 beside or behind the urinary aperture. 



The subdivisions of the osseous fishes are so numerous, and 

 they contain so many families, that it will be sufficient to run 

 over the more important sub-orders, and to mention the more 

 familiar examples of each. 



SUB-ORDER A. MALACOPTERI, Owen ( = Physostomata, Miil- 

 ler). This sub-order is defined by usually possessing a com- 

 plete set 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 always communicates with the oesophagus by means of a 

 duct, which is the homologue of the windpipe. The skin is 

 rarely naked, and is mostly furnished with cycloid scales ; but 

 in some cases ganoid plates are present. 



This sub-order is one of great importance, as comprising many well- 

 known and useful fishes. It is divided into two groups, according as 

 ventral fins are present or not. In the first group Apoda there are 

 no ventral fins; and the most familiar examples are the common Eels 

 of our own country. The Eels (Mur&nida) have an elongated, almost 

 cylindrical body, with the scales deeply sunk in the skin, and scarcely ap- 

 parent. A swim-bladder is present, and the operculum is small and mostly 

 enveloped in the skin. More remarkable, however, than the ordinary 

 Eels is the Gymnotus electricus, or great Electric Eel (fig. 262), which in- 

 habits the marshy waters of those wonderful South American plains, the 

 so-called " Llanos," and which shares with various fishes of diverse affinities 

 (the Torpedo, the Malapterurus electricus, &c.) the power of generating 

 electricity by means of special organs. 



The second group of the Malacopteri is that of the Abdominalia, in which 

 there are ventral fins, and these are abdominal in position. Space will not 

 permit of more here than merely mentioning that in this section are con- 

 tained amongst others the well-known and important groups of the Chtpeida 

 (Herring tribe), the Pikes (Esocida>\ the Carps, Barbels, Roach, Chub, 

 Minnow, c. (Cyprinida:), and the Salmonidce, comprising the various 



