CHARACTERS OF VERTEBRATE ANIMALS. . 9 



development closely corresponding with that of the pancreas. Lac- 

 teal vessels convey the nutrient fluid to the veins, and thus it reaches 

 the heart. 



The central organ of circulation, always present, and of a compact 

 muscular character, always below or anterior in position to the ali- 

 mentary tube and nervous axis, is situated towards the fore-part of 

 the body, most commonly in a compartment distinct from the abdo- 

 men, where it is suspended in a special bag or pericardium {fig. 1. 

 K). The blood is red in all the Vertebrated animals, and the colouring 

 matter is contained in microscopic discoid cells, of an oval or circular 

 form {fig. 4.). The whole or part of the circulating fluid is trans- 

 mitted directly from the heart to the respiratory organ {fig. 1. lg\ 

 The respiratory medium, whether air or water, is admitted to the 

 respiratory organ by the mouth. From this organ the arterial blood 

 is sent, sometimes directly, sometimes after a second return to the 

 heart, or in both ways, to the rest of the system ; but the breathing 

 organs are never developed, as we saw in many of the Inverteln*ata, 

 from the returning venous channels. 



The venous blood in the lower Vertebrata is submitted to the 

 depui'ating influence of the kidneys ; but in the higher Vertebrata 

 these de-azotising glands {li) are supplied exclusively by arteries. 

 A part of the venous blood in all Vertebrata circulates through the 

 liver, as through a second and subordinate lung, before it linally 

 reaches the heart. 



The system for perpetuating the species is not complete in any 

 Vertebrated animal ; that is, the generative organs are divided be- 

 tween two individuals, there being no natural Hermaphrodite in this 

 sub-kingdom. Every Vertebrate embryo soon takes on its special 

 and determinate sexual character, and ends a perfect male or per- 

 fect female — a fertiliser or a producer. 



The instinctive sense of dependence upon another, manifested by the 

 impulse to seek out a mate, — which impulse, even in fishes, is some- 

 times so irresistible that they throw themselves on shore in the pur- 

 suit, — this first step in the supercession of the lower and more general 

 law of individual or self preservation, although not first introduced 

 at the Vertebrate stage of the animal series, is never departed from 

 after that stage has been gained. To this sexual relation is next 

 added a self-sacrificing impulse of a higher kind, viz. the parental 

 instinct. As we rise in the survey of Vertebrate phenomena, we sec 

 the entire devotion of self to offspring in the patient incubation of 

 the bird, in the unwearied exertions of the Swift or the Hawk to 

 obtain food for their callow brood when hatched ; in tlie bold de- 

 monstration which the Hen, at other times so timid, will make to 

 repel threatened attacks against her cowering young. 



