VERTEBEATA. 7 



f. Skull imperfectly developed and without jaws ; paired fins un- 

 developed, with no shoulder girdle or pelvic elements ; a single 

 median nostril ; gills purse-shaped; skin naked ; skeleton car- 

 tilaginous Cyclostomi, D. 



ff. Skull well developed, and with jaws; shoulder girdle and pelvic 



elements developed; nostrils not median. 

 g. Limbs developed as rayed fins (rarely abortive) ; rayed fins nor- 

 mally present on the median line of the body ; respiration 

 throughout life by means of gills ; lungs usually not developed, 

 d Pisces, E. 



gg. Limbs not developed as rayed fins, but, if present, having the same 

 skeletal elements as in the higher vertebrates ; respiration in the 

 adult chiefly accomplished by means of lungs, the gills usually 



not persistent ; skin usually naked Batrachia, F. 



ec. Respiration performed throughout life by means of lungs, the gill 



slits disappearing before birth. 



h. Mammary glands not present; diaphragm incomplete; a single 



occipital condyle; oviparous (or sometimes ovoviviparous), 



the young hatched from a rather large egg. (Sauropsida.) 



i. Exoskeleton developed as scales or bony plates; blood cold; 



heart with three (rarely four) cavities. . . Reptilia, G. 



if. Exoskeleton developed as feathers; blood warm; heart with 



four cavities AvES, H. 



AA. Mammary glands present; the young developed within the body 

 from a minute ^^^ (except in the Monotremata)^ and nourished 

 for a time after birth by milk secreted in the mammary glands ; 

 exoskeleton developed as hair; two occipital condyles; dia- 

 phragm complete ; heart with four cavities ; blood warm. 



Mammalia, I. 



Of tliese classes, the Tunicata (A) and the Enteropneusta (B) are 

 excluded from the plan of the present work. The Tunicata are all 

 marine forms, of small size, the larger species being familiarly 

 known as " Sea Squirts," " Sea Peaches," and " Sea Pears " ; but 

 the most of them are without common names. A considerable 

 number of species, representing several families, are found on our 

 Atlantic coast. The Enteropneusta consist of the single genus 

 Balanoglossus, a worm-like creature, of which two or three species 

 are found on our coasts. They reach a length of six to twelve 

 inches. They have been considered as worms having possible 

 affinities with the Echinoderms, but the recent studies of Mr. 

 WiUlam Bateson seem to show conclusively that their place is 

 among the Chordata. 



Leaving these groups aside, we take up 



