224 ZOOLOGY sect. 



branchial aperture is very small. The exoskeleton consists of 

 bony plates arranged segmentally. 



This is also a very small sub-order, including only the Sea- 

 horses (Fig. 881), Pipe-fishes and their allies. 



Sub-orders h—f are frequently grouped together as Physodisti, 

 distinguished from Physostomi by the closed air-bladder. 



Systematic Position of the Example. 



Salmo fario is one of several species of the genus Salmo, belong- 

 ing to the family Salmonidcc, of the sub-order Physostomi and the 

 order Teleostei. 



The absence of a spiral valve and of a conus arteriosus, the 

 presence of a bulbus aortse, and the decussation of the optic 

 nerves indicate its position among the Teleostei. It belongs to 

 the Physostomi in virtue of possessing a pneumatic duct, none but 

 jointed fin-rays, and abdominal ventral fins. The characters which 

 place it among the Salmonidse are the presence of an adipose fin 

 and of pseudobranchia', the absence of oviducts, and the fact that 

 the maxilla enters into the gape of the mouth. The genus 

 Salmo is distinguished by its small scales, well-developed conical 

 teeth, absent on the pterygoids, a short ventral fin with fewer 

 than fourteen rays, numerous pyloric appendages, and compara- 

 tively large ova. The distinctive characters of the various species 

 of Salmo depend upon comparatively minute points, such as the 

 relative proportions of various parts, and are often difficult of 

 determination owing to individual variations correlated with 

 different environments. In S. fario the posterior margin of the 

 operculum is evenly curved, the maxilla is longer than the snout, 

 and the vomerine teeth are in a double series and persist throughout ' 

 life. 



S. General Organisation. 



External Form. — The topical form of the Teleostomi is very 

 fairly represented by that of the Trout (Fig. 854) — a long, com- 

 pressed body, nearly half of which is formed by the tail, pointed 

 anterior and posterior ends, a large vertical tail-fin, a head of mode- 

 rate size, and a terminal mouth. Such a form is eminently fitted for 

 rapid progression through the water. But from this characteristic 

 fish-form there are many striking deviations. The body may be 

 greatly elongated and almost cylindrical, as in the Eels ; or of great 

 length and strongly flattened from side to side, as in the Ribbon- 

 fishes ; or the head may be of immense proportional size and 

 strongly depressed, as in certain shore-fishes, such as the "Fishing- 

 frog " ; or, as in the beautiful Reef-fishes, the whole body 

 may be as high as it is long. The mouth sometimes has a 



