12 INTRODUCTORY LECTURE. 



damental group of Vertebrated animals, are never destined to quit the 

 watery medium ; these constitute the class of Fishes. A few species 

 retain the primitive vermiform type, and have no distinct locomotive 

 members ; and these members, in the rest of the Piscine class, are 

 small and simple, rarely adapted for any other function than the pro- 

 pulsion or guidance of the body through the water. The form of the 

 body is, for the most part, such as mechanical principles teach to be 

 best adapted for moving with least resistance through a liquid 

 medium. The surface of the body is either smooth and lubricous, 

 or is covered by closely imbricated scales, rarely defended by bony 

 plates or roughened by hard tubercles ; still more rarely armed with 

 spines. 



The central axis of the nervous system presents but one partial 

 enlargement, and that of comparatively small size, at its anterior ex- 

 tremity, forming the brain, which consists of a succession of simple 

 ganglionic masses ^fig. 46.), most of them exclusively appropriated 

 to the function of a nerve of special sense. The power of touch can 

 be but feebly developed in fishes. The organ of taste is a very in- 

 conspicuous one, the chief function of the framework supporting it, 

 or the hyoidean apparatus, relating to the mechanism of swallowing 

 and breathing. 



Of the organ of hearing there is no outward sign ; but the essential 

 part, the acoustic labyrinth, is present, and the semicircular canals 

 largely developed within. The labyrinth is without cochlea, and is 

 rarely provided with a special chamber, but is lodged, in common 

 with the brain, in the cranial cavity. The eyes are usually large, 

 but are seldom defended by eyelids, and never served by a lachrymal 

 organ. The alimentary canal is commonly short and simple, with its 

 divisions not always clearly marked, the short and wide gullet being 

 hardly distinguishable from the stomach. The pancreas, for the most 

 part, retains its primitive condition of separate cjBcal appendages to 

 the duodenum. The heart consists essentially of one auricle and one 

 ventricle, receiving the venous blood, and propelling it to the gills ; 

 whence the circulation is continued over the entire body in vessels 

 only, which are aided by the contraction of the surrounding muscular 

 fibres. 



The blood of fishes is cold ; its temperature being rarely elevated 

 above that of the suri'ounding medium. The coloured discs are 

 sometimes subcircular i^fg. 4. g\ sometimes subelliptical {K) or ellip- 

 tic : comparatively large, but not the largest amongst vertebrate 

 animals. 



The primordial elongated I'enal glands are persistent, and secrete 

 the urine from venous blood. 



