xiii PHYLUM CHORDATA 543 



The single aortic arch, situated in all Mammals on the left side, 

 varies greatly in the way in which it gives off the main arterial 

 trunks. Sometimes a single large trunk passes forwards from the 

 arch of the aorta and gives rise to both carotids and both sub- 

 clavians. Sometimes there are two main trunks right and left 

 innominate arteries each giving rise to the carotid and subclavian 

 of its own side. Sometimes there is a right innominate giving 

 off right carotid and right subclavian, the left carotid and left 

 subclavian coming off separately from the arch of the aorta, or, 

 as in the Rabbit, an innominate may give origin to the right 

 subclavian and both carotids, the left subclavian coming off 

 separately. 



In Monotremes and Marsupials, in most Ungulates, and in 

 the Rodentia, Insectivora, and Chiroptera, both right and left 

 pre-cavals persist ; in the others the left aborts, its vestige giving 

 rise to the coronary sinus. In the Monotremes the openings of 

 all three cavals are provided with valves, only vestiges of which 

 exist in the other groups. In the Monotremes all the pulmonary 

 veins open by a common trunk. In the Metatheria and Eutheria 

 the four veins sometimes open separately, sometimes the two veins 

 of each side unite to form a single lateral trunk. 



The following are some of the principal variations in the struc- 

 ture of the heart which occur in the different groups of Mammals. 

 In the Monotremes there is a deep fossa representing the fossa 

 ovalis in the auricular septum. The tricuspid valve in Ornitho- 

 rhynchus consists of two membranous and two fleshy portions ; the 

 mitral valve is wholly membranous. In Echidna the tricuspid 

 valve is completely membranous, and consists of two portions a 

 larger and a smaller. In the Marsupials the fossa ovalis and 

 annulus ovalis are absent ; in the uterine foetus of the Kangaroo 

 the auricles communicate by a fissure, but all trace of this becomes 

 lost before the adult stage is reached. 



In the Cetacea, Eustachian and Thebesian valves are both 

 absent. In some of the Cetacea the apices of the ventricles 

 are separated by a slight depression. In the Sirenia there is 

 a corresponding, bub much deeper and wider, cleft, so that the 

 apex of the heart is distinctly bifid. 



In the Ungulata, Eustachian and coronary valves are both 

 absent ; in some there is a cartilage or a bone the os cordis 

 often double, at the base of the heart. The Eustachian valve is 

 absent in most of the Carnivora. In the Pinnipedia, an aperture of 

 communication between the auricles often persists in the adult. 



The organs of respiration resemble those of the Rabbit in 

 the general features mentioned on p. 437. 



In the Cetacea, the epiglottis and arytenoids are prolonged to 

 form a tube, which extends into the nasal chambers, and is em- 

 braced by the soft palate, so that a continuous passage is formed, 



