296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



the marsupials a pouch is usually present into which the very small 

 young find their way, attaching themselves to one of the mammae. 

 In some bandicoots, as Perameles, a placenta is present, although the 

 young are born in as undeveloped a condition as in the other 

 marsupials. 



ANALYSES 



In the preceding reviews of the features of the several vertebrate 

 classes it is evident in each case that some of the features are distinctive 

 and diagnostic, fundamental attributes of the classes, while others 

 represent specializations in various directions, attributes of what may 

 be called fringe or peripheral types. The fundamental characters of 

 the vertebrate classes seem to be the following : 



Fishes. — Aquatic animal feeders of fresh or saline waters subsisting 

 mainly on insects and other invertebrates, especially crustaceans ; with 

 a covering of scales, naked, or with dermal scutes ; body temperature 

 uncontrolled, approximating that of the surrounding water ; breath- 

 ing by gills, sometimes supplemented by lunglike structures or, rarely, 

 lungs ; not dependent on bright illumination ; oviparous, the fertiliza- 

 tion usually external, the eggs soft, usually with a gelatinous cover- 

 ing or in a gelatinous matrix, sometimes in a tough capsule, attached, 

 less commonly scattered or pelagic; commonly with a larval stage 

 widely different from the adult; a few are viviparous. 



Amphibians. — Amphibious animals of exclusively fresh waters or 

 very humid regions; animal feeders subsisting chiefly on insects, 

 though some are detritus feeders, especially in the larval stages; 

 naked, with a moist skin; body temperature uncontrolled, approxi- 

 mating that of the surroundings; breathing by external or covered 

 gills or by lungs, or by both at different stages, and also through the 

 skin; not dependent on bright illumination, and largely nocturnal; 

 oviparous, the eggs fertilized externally, soft, usually in water, with 

 a gelatinous covering or in a gelatinous matrix ; mostly with a fishlike 

 larval stage breathing with gills in water, later more or less terrestrial, 

 breathing with lungs ; in a few the young are born with the adult form. 



Reptiles.— Terrestrial animal feeders subsisting mainly on insects, 

 the largest on vertebrates ; with a complete body covering of scales ; 

 the skin is dry ; body temperature uncontrolled, approximating that 

 of the surroundings ; breathing by internal lungs ; highly dependent 

 on bright illumination, especially from the two ends of the spectrum, 

 chiefly diurnal, but the geckos mainly nocturnal and the venomous 

 snakes feeding mostly at night; oviparous, with internal fertilization, 

 the eggs with a tough or hard and rigid shell; some are viviparous; 

 young at birth resembling the adults. 



Birds.— Predominantly terrestrial animal feeders, subsisting chiefly 

 on insects, but many are vegetarians, at least as adults ; with a body 



