362 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



two striking differences between the eye of the teleost and that of a 

 Mammal such as Man in which (i) the eye when at rest is focused on 

 distant objects and (2) accommodation (change of focus) is brought about 

 not by displacement of the lens but by a change in its curvature. 



The olfactory organ of the Teleost has become shifted on to the 

 dorsal surface of the snout and its external opening is usually divided 

 by a cross bridge into two. For comparison of these two openings 

 with those in the higher vertebrates (see p. 389) it must be borne in mind 

 that the shifting on to the dorsal surface causes a reversal in the posi- 

 tion of the openings, that which is anterior in the original position coming 

 to be in the teleost posterior. 



The otocyst has lost its communication with the exterior. The large 

 otolith in the saccule is usually very hard and dense and in many cases 

 shows distinct zones of growth like those of the scales so that it may be 

 used for age-determination. 



The group Teleostei includes as already mentioned the great majority 

 of modern fishes : they constitute one of the predominant groups of 

 vertebrates existing at the present day and they represent the highest 

 evolutionary attainment in the way of specialization for an aquatic 

 existence. The group is subdivided into a large number of families of 

 which a few of the more important must be mentioned. 



The Mormyridae are a group of African fresh-water fishes noteworthy 

 in two respects : (i) the brain is of greater proportional size than in any 

 other lower vertebrates and (2) the group includes two genera (Mormyrus 

 and Gymnarchus) possessing small electric organs in the tail region. 



The Clupeidae include a number of important food-fish such as the 

 Ik-mug, Sprat, Pilchard, Anchovy and Shad. "Sardines" are young 

 Pilchards, and " Whitebait " consists mainly of the fry of Herrings. A 

 point of much economic importance is that the eggs of the Herring 

 instead of floating freely in the water, as is the case with most marine 

 lood-fish. are "demersal," i.e. they lie on the bottom, adhering to one 

 another and to stones so that they are liable to destruction by heavy 

 trawls scraping over the bottom of the sea. 



The Salmonidae include the various kinds of Salmon, Trout, Charr, 

 \Vhitefish, Grayling and Smelt. Many of them have taken to fresh 

 water either as permanent residents or as temporary visitors for the 

 purpose of spawning (Salmon, " Sea-trout "). The name anadromous is 

 applied to fish which ascend rivers in order to spawn. 



''harannidae include a great variety of fresh-water fish of the 

 Atri< an ,md South American continents. Amongst them are the Salmon- 



