456 



THE CAT. 



[CHAP. 'xin. 



class embraces all crocodiles, lizards, serpents, and tortoises, with 

 extinct forms more or less allied to these. The province BRANCHIATA* 

 is the third and last. It consists of two classes : the class Batra- 

 chiai.e., the class of frogs, toads, efts, and their allies and the 

 class Pisces, which includes all fishes. 



Our present task, then, is to see what is implied in saying that 

 the cat is a " beast " or a " mammal." To see this we must know 

 its relations, simply as a mammal, to the other forms of vertebrate 

 life, i.e., to the groups Pisces, Batrachia, Branchiata, Reptilia, 

 Aves, and Monocondyla, and to all the non-mammalian vertebrates 

 taken together. 



If, to help us in our comparisons, we examine any ordinary fish, 

 such as the cod (Gadus), we see in it an animal with a large, neckless 

 head and a long and powerful tail, with a membranous production 

 from the back (a dorsal fin), one beneath the tail (an anal fin), and 

 two pairs of fins placed laterally, ventrally and far forwards, and 

 termed respectively the pectoral and ventral fins. The body of the 

 fish is clothed with scales, and exhibits an undulating mark running 

 from before backwards (from head to tail) on each side, called the 

 lateral line. The tail ends in a membranous, vertical expansion, or 

 caudal fin, which is supported (as are all the other fins) by many 

 delicate but firm fibres within its substance, called fin-rays. The 

 nostrils do not open into the mouth posteriorly, but each has two 

 openings on the exterior of the snout. The orbits do not com- 

 municate with the nares by any lachrymal tube, nor the ears with 

 the mouth by any Eustachian tube, but there is a large aperture on 

 each side behind the head, which leads into a chamber wherein are 

 a number of membranous plates, or " gills," attached to the outside 

 of a series of arches between which are clefts leading into the 

 alimentary canal just behind the mouth. These gills are breathing 

 organs, adapted to aid the needful respiratory gaseous exchange 

 between the blood within them and the atmospheric air which is 

 dissolved in the water the animal inhabits. There is a perfectly- 

 closed air-bladder just beneath the vertebral column, but there are 

 no lungs and no pulmonary arteries. The aorta divides as in the 

 embryo cat into a series of arches which ascend the sides of the 

 pharynx to reach the dorsal aorta, but each artery is, on its way 

 upwards, broken up into a network of minute capillary vessels 

 within the membranes or leaflets of the gills. Thus the blood leaves 

 the heart in an impure or venous condition, and, having gone thence 

 to the aquatic respiratory organs or gills it does not return directly 

 to the heart, but passes up in an arterial condition to the dorsal 

 aorta. Consequently neither a second auricle nor a second ventricle 

 is required, and neither exists the heart consisting, as in the 

 embryo cat, only of a single auricle and ventricle, the venous blood 

 passing into the former from a " sinus venosus," and being given 



* A term referring to the all but uni- 

 versal presence of gills (at one time of 



life if not permanently) in all Batrachians 

 and Fishes. 



