ANATOMY OF VERTEBRATES. 



the bony or gristly levers on which they act. The limbs do not 

 exceed two pairs. The sexes are distinct, and the individual 

 is developed directly from an impregnated ovum. Under the 



vertebrate plan of structure animals 

 grow to a greater size and live a 

 longer time, than under any of the 

 invertebrate plans. 



^^^^1 3. Piscine modification. All 

 pleurapophysis vertebrates, during more or less of 

 their developmental life-period, float 

 in a liquid of similar specific gravity 

 to themselves. A large proportion, 



constituting the lowest organised and 



~ ~ 



first developed forms of the pro- 

 vince, exist and breathe in water, 



and are called ( fishes.' Of these a few retain the primitive 

 vermiform condition and develope no limbs : in the rest they are 

 ( fins,' of simple form, moving by one joint upon the body, rarely 

 adapted for any other function than the impulse or guidance 



8 



||neural spine 



zygapophysis J||% neurapophysis 

 cliapophysis _.f^ ^_ 



2~~rc | LI " '''"''vor*- 



parapopliysis ^i^_^,_x3 



ijJiff hremapophysis 

 zygapophysis :;F O 



ll hajmal spine 

 Ideal typical vertebra. CXLV. 



d 



g 



Blood-discs, each magnified 300 diameters linear, a, Man ; &, Musk-deer ; c, Goose ; d, Crocodile ; 



e, Frog ; /, Siren ; g, Cod-flsh ; 7i, Skate. CXLV. 



of the body through the water. The shape of the body is usually 

 such as is adapted for moving with least resistance through a 

 liquid medium. The surface of the body is either smooth and 

 lubricous, or is smoothly covered by overlapping scales, is rarely 

 defended by bony plates or roughened by tubercles, still more 

 rarely armed with spines. 



The neural axis presents but one local enlargement, at the fore 

 end, forming the ( encephalon ; ' it is small, and consists of a suc- 

 cession of simple ganglionic masses, most of which are appro- 

 priated to the function of a nerve of special sense. Touch is 

 feebly exercised, and an organ for that sense rarely developed. 



