NATURAL HISTORY. 325 



" Forthwith the sounds and seas, each creek and bay, 

 "With Me innumerable swarme, and shoales 

 Of fish, that with their ftnnes and shining scales 

 Glide under the greene wave." MII.TOX 



the surrounding medium ; and the changes of temperature "which 

 they thus experience have great influence upon all their 

 functions. 



A warmth of from 105 to 120 is soon fatal to most of these 

 animals ; and cold tends to depress all their natural operations. 

 In winter, most of them could no longer digest the food introduced 

 into their stomach, and do not take nourishment. Their respiration 

 also diminishes in a most remarkable manner. Thus, during the 

 cold season, the action of the air on the skin is sufficient for 

 the maintenance of the life of the frog ; and the lungs of one 

 of these animals may be removed without producing asphyxia ; 

 whilst in summer they have need not only of the pulmonary 

 respiration, but also of their cutaneous ; and death soon occurs 

 when the air does not act on the skin, or is excluded from 

 the lungs. 



CLASS IV. PISCES. 



ORDER I. LEPTOCARDIA.* 



1038. Why is the first order of fishes called leptocardia ? 



From two Greek words signifying small and heart, with reference 

 to the rudimentary formation of the heart, which, indeed, is said 

 to be absent, and to consist entirely of the contractile power 

 of the arteries. 



1039. This order includes only a single small fish, which rarely attains a length 

 of two inches, and which presents so many remarkable characters that its position 

 in classification has been much disputed. This is the Amphioxus lanceolatus, a 

 little, slender, transparent creature, found on sandy coasts in various parts of the 

 woz-ld. Its body is of the lamprey form, with a narrow membranous border. 

 The vertebral column is represented by a gelatinous cord, wlvch supports the axis 

 of the nervous system; the latter terminates anteriorly by a rounded extremity, 

 without any signs of a brain. The head bears a pair of eyes, which are connected 

 with the end of the nervous axis by short filaments, and there is an apparent 



The classification here pursued combines the systems of Cuvier and Agassiz, 

 a& blended by M tiller. 



