CHARACTERS OF THE SUB-KINGDOMS. 15 



To the various 'parts of this skeleton the muscles are 

 attached. 



The nervous system of the Vertebrata is very character- 

 istic. An invertebrate animal consists only of a single 

 tube, which contains the nervous system as well as the 

 organs of circulation and digestion. If a vertebrate 

 animal, however, be cut in two, it will be seen to consist 

 of two tubes, an upper and a lower. The upper tube is 

 formed by two bony arches which proceed from the 

 centre or body of the vertebra or joint of the backbone. 

 These are termed the neural arches (Or. neuron, a 

 nerve), and form the canal in which the spinal cord is 

 enclosed. This cord is connected with the brain, which 

 is contained in the skull. These form the main mass 

 of the nervous system. 



The lower and much larger tube contains the alimen- 

 tary canal and other digestive organs, the organs of cir- 

 culation, and what is called the " sympathetic " nervous 

 system which seems to correspond with the nervous 

 system of the Invertebrata. 



Some Yertebrata, as the serpents, &c., are destitute of 

 limbs ; and some, as the whales, possess only one pair. 

 Generally, however, there are two pairs present, and no 

 vertebrate animal has more than four limbs. 



They have all a complete alimentary or intestinal canal 

 provided with two openings, terminating respectively in 

 a mouth and anus or vent. This canal is always com- 

 pletely shut off from, the general cavity of the body. 

 There is generally an oesophagus or gullet, and a stomach 

 with large and small intestines. 



The heart is formed of a variable number of cavities, 

 called auricles and ventricles. Mammalia, or animals 

 which nourish their young with a fluid called milk, and 

 birds, have two auricles and two ventricles ; reptiles and 

 amphibia (frogs, toads, &c.), two auricles and one ven- 

 tricle ; fishes, one auricle and one ventricle. 



The blood is red, with the exception of one species of 

 fish, the lancelet, which has colourless blood. The colour 



