HIGHER CHORDATES 



293 



swimming; whales and manatees lack hind limbs; 

 paired or single nostrils lead to mouth; respiration 

 by lungs; body temperature internally regulated; 

 sexes separate, almost always an embryonic stage 

 nourished by mother, females with milk-producing 

 mammary glands for suckling young. 



Subclass PROTOTHERIA (Egg-laying Mammals) 



Diagnosis: specialized but primitive egg-laying 

 mammals (egg-laying probably was riot present in 

 earliest mammals); possess many features of their 

 reptilian ancestors; egg heavily yolked, with a flexible 

 shell, and early development reptile-like; teeth only 

 in young, adults with a horny beak but without an 

 external ear pinna; mammary glands without nip- 

 ples; includes one species of platypus and four species 

 of spiny anteaters, or echidnas, in two genera; the 

 five species constitute the single Order Monotremata 

 (Figure 16.25). 



The platypus grows to 2 feet long. Head flattened, 

 with a sensitive, rubbery, duck-like bill; body chunky; 

 tail flattened, beaver-like; feet webbed, with heavy 

 claws; hair dense and velvety; semiaquatic; under- 

 water predator on invertebrates, prey masticated with 

 horny tooth plates; construct burrows in stream or 

 lake banks, females construct longer burrows (up to 

 60 feet) with a side nesting chamber; females lay and 



incubate one to three eggs, which hatch in 7 to 10 

 days; young nurse by lapping milk from mother's 

 hair; young enter water in about 4 months; found in 

 eastern Australia. 



The echidnas grow to about 20 inches long. Body 

 rounded; head with an elongate, cylindrical beak; 

 tongue can be extended to lap up insects, jaw with 

 horny ridges to grind insects against ridged palate; 

 tail a stump; legs short, three to five toes are clawed; 

 hair coarse, some modified as spines; terrestrial in- 

 sect predators; do not burrow; female lays one egg, 

 carried in a pouch that is present during breeding 

 season (pouch not the same as that of marsupials); 

 young remain in pouch for a few weeks; young nurse 

 by lapping milk from mother's hair; found in Aus- 

 tralia, Tasmania, and New Guinea. 



Subclass THERIA (Viviparous Mammals) 

 Diagnosis: females bear living young. 



Infraclass Metatheria (Marsupials) 



Diagnosis: females usually with an abdominal 

 pouch (marsupium) or folds surrounding nipples; 

 embryos usually develop without a placenta; newborn 

 are premature "embryos" that crawl to pouch, attach 

 to a nipple, and stay until fully developed; after devel- 



nostri 



TACHYGLOSSUS 



ORNITHORHYNCHUS 



Figure 16.25 The two living genera of monotremes, Orniihorhynchus, the plotypus, and Tachy- 

 g/ossus, the spiny onteoter. (From Malcolm Jollie, Chordaie Morphology, Reinhold Publishing 

 Corp., New York, 1962.) 



