SIMPLIFICATION OF ORGANIZATION. 89 



The diaphragm ends with the mammalia : so that the 

 thorax and abdomen are not distinct in any other animals. 



The circulation is reduced in reptiles to the single state, 

 and is carried on by one auricle and ventricle. 



Warmth of the blood — that is a temperature of that 

 fluid considerably elevated above the surrounding medium 

 — belongs only to mammalia and birds ; and the red colour 

 of the same fluid is confined, with one small exception, to 

 the vertebral animals. 



Organs of voice end in reptiles ; not existing in fishes. 



Viviparous generation, with its attendant process of suck- 

 ling the young, is confined to the mammalia ; and is after- 

 wards succeeded by the more simple oviparous form. 



Urinary organs end with the mammalia, many of which 

 have no bladder ; as birds, some fishes, and reptiles. 



The absorbent system terminates in the vertebral depart- 

 ment ; of wliich only the mammalia and birds possess lym- 

 phatic glands. 



The mollusca present an organization very much reduced 

 in the number of its parts, and very imperfect in all respects,- 

 when compared to that of the vertebral animals. They 

 have no skeleton to lod^e tlie nervous system, and form the 

 centre of motions ; no separate receptacles for the various 



on the abdominal instead of the dorsal aspect of the body, points out a great 

 difference between it and the spinal marrow of the four vertebral classes. 

 The spiders and phalangia, which in other respects are allied to other insects, 

 have no such cord, but, like the mollusca, single ganglia, not placed in a 

 straight direction one behind tlie other. Tiie true spinal marrow is only 

 found in mammalia, birds, reptiles, and fishes." Biologle, b. v. p. 331, 332. 

 ''In this view, the representation that the great sympathetic nerve belongs 

 only to red-blooded animals, must be deemed incorrect. This very nerve is 

 the most general, the original of all nerves ; but it is variously modified in 

 the different classes. In worms and insects there are merely vertebral gang- 

 lia, without the coeliac ganglia of mammalia and birds; in the acephalous 

 mollusca there are the latter, without the former ; in the cuttle-fish and snails 

 there are single ganglia of both kinds. All these lower animals have no spi- 

 nal marrow ; fishes and reptiles have one, and also vertebral ganglia; but the 

 cceliac ganglia cither do not exist in them, or are not so developed as in birds 

 and mammalia." Ibid, 331-5. 



