Comparative Morphology of Ghordates 



Subclass i. Monotremata 



The monotremes resemble reptiles in many anatomic features — the 

 persistent cloaca; the completely separate oviducts with no differ- 

 entiated vaginal regions, opening independently into the cloaca; 

 ureters opening into the cloaca instead of directly into the bladder: the 

 incompletely closed urethral groove of the cloacal penis: the pectoral 

 girdle with a strongly developed coracoid bone; a distinct episternum 

 (or interclavicle, a dermal bone) projecting forward from the sternum; 

 the pair of epipubic ("marsupial") bones; the columella-like form of 

 the stapes; the brain without a corpus callosum. Also, they are im- 

 perfectly homothermous, the temperature of the body being lower 

 than in other mammals and varying somewhat with the external 

 temperature. 



The question has been raised as to whether Archaeopleryx was a 

 feathered reptile or a reptilian bird. A similar question may be raised 

 about monotremes — are they reptiles with hair and mammary glands, 

 or are they very reptilian mammals? In the case of Archaeopleryx, it 

 was considered that feathers make it a bird, and so, in the case of 

 monotremes, more weight must be given to the possession of new and 

 definitely mammalian features than to mere retention of reptilian 

 features. 



Ornithorhynchus (Fig. 532), about as large as a common cat. has 

 a stout body, a rather short and dorso vent rally flattened tail, and a 

 small head with small laterally directed eyes and no auditory pinnae. 

 (This absence of pinnae is not necessarily a primitive reptilian feature. 



Fig. 532. Ornithorhynchus. (Courtesy, Vogt and Specht: *" Die Saugetiere in Wort 

 u i id Rild," Munich, V. Bruckmann-Verlag.) 



