728 Comparative Morphology of Chordates 



the duckbill, there are no auditory pinnae or, at most (in one species), 

 only very poorly developed pinnae. The mammary organs resemble 

 those of the duckbill except in that each mammary area is depressed 

 to form a shallow pocket at whose bottom open the milk-ducts. \ few 

 long hairs project from the pocket, possibly serving to guide the exud- 

 ing milk into the mouth of the young animal, whose beak is inserted 

 into the pocket. During the period of rearing young, a fold of abdominal 

 skin in the vicinity of the mammary pockets forms a temporary mar- 

 supium in which the egg is incubated and the young retained for a 

 time after hatching. The brain is better developed than that of the 

 duckbill. It is relatively larger and its cerebral cortex is well con- 

 voluted, in contrast to the quite smooth cortex of the duckbill. 



Echidna avoids water and eats ants, having the long sticky tongue 

 characteristic of anteaters. The strong claws are used for digging into 

 anthills and excavating deep burrows. 



Subclass ii. Marsupialia 



The marsupials are intermediate between monotremes and pla- 

 cental in some respects — size of egg and duration of the period of 

 intrauterine development ; retention of merely the shallow ectodermal 

 region of the embryonic cloaca ; a vaginal region differentiated in each 

 oviduct but the two vaginas not fused into a single median vagina; 

 presence (in at least some marsupials) of an extensive cartilaginous 

 coracoid in the embryo, although only its proximal end persists and 

 ossifies to form the ventral coracoid process of the scapula. In some 

 features they resemble monotremes — presence of epipubic bones; ab- 

 sence of corpus callosum. In many features they are like placentals — 

 ureters opening directly into the bladder; complete enclosing of the 

 cloacal urethral groove to form the urethral canal of the penis; absence 

 of episternum (interclavicle) ; mammary glands with well-developed 

 nipples. Their most unique characteristic is the marsupium. Whether 

 the temporary egg-pouch of Echidna is homologous with this mar- 

 supium is doubtful, and in placentals there are, at most, only some 

 possible embryonic vestiges of the marsupium. However, just as there 

 are some "vertebrates" without a vertebral column, so there are a few 

 marsupials with no marsupium. 



Subclass hi. Placentalia 



The highly developed placenta is the preeminent anatomic char- 

 acteristic of placental mammals. Otherwise, they are characterized by 

 complete absence of the marsupium (except for possible embryonic 

 vestiges) and of the epipubic bones; by reduction of the coracoid 



