268 AN INTRODUCTION TO ZOOLOGY 



important in locomotion we find the pectoral girdle is poorly deve- 

 loped. On the other hand, owing to the greater use of the hind limbs, 

 the pelvis girdle is large and securely attached to the backbone. 



The coelom is completely divided into two cavities, by the 

 development of a partly membranous, largely muscular arched 

 partition, the diaphragm, situated in the region of the hinder end 

 of the ribs. Thus we can recognise an anterior, pleural cavity, 

 lying in a part of the body termed the thorax, containing the lungs 

 and having within it the pericardial cavity and a posterior one, 

 situated in the abdomen, termed the peritoneal cavity, which contains 

 the rest of the viscera. The movements of breathing differ com- 

 pletely from those in Rana. Air is drawn into the lungs by the 

 enlargement of the thorax either by the movement of the ribs or the 

 flattening of the arch of the diaphragm, or more commonly both 

 combined. 



The kidneys are metanephroi and the ureters open directly into 

 the bladder, not serving for the passage of the sperms during any 

 part of their course. In the female, the lower ends of the oviducts 

 are modified to form characteristic structures, the uteri, which in 

 some cases fuse together forming a single uterus. The ova (save in 

 Ornithorhynchus and Echidna) are minute and practically yolkless, 

 although their structure and mode of development give distinct 

 indications that they have been derived from eggs with a large 

 amount of yolk such as we find in reptiles. Save in the forms just 

 mentioned the eggs are never passed to the outside, but undergo 

 development within the uterus to the walls of which they are attached 

 for a short time (Marsupials) or a relatively long time (as in higher 

 mammals, Eutheria) by a very characteristic organ termed the 

 placenta. This is composed of an intimate union of tissues from 

 both mother and embryo and serves for the transference of food, 

 oxygen, and excretory products. After birth the young animals 

 are quite incapable of obtaining food for themselves and are 

 dependent upon the milk secreted by the mammary glands of the 

 mother. 



The heart of the mammal is completely divided into two sides 

 by the interatrial septum and the further development of a median 

 partition, the septum ventriculorum, in the ventricle, and the two 

 sides have no means of intercommunication. More than that, in 

 order that the blood in a ventricle may return to it again, it is necessary 

 for it to leave the heart twice, once to go to the tissues and return 

 and once to go to the lungs and back. We refer to this condition 

 as a complete double circulation. The heart contains only four 

 chambers, two auricles, or better atria, and two ventricles, there being 

 no distinct sinus venosus and no conus arteriosus. A hepatic 



