22 COMPARATIVE ANATOMY. 



ent; blood corpuscles, red and always nucleated; nasal sacs 

 communicate with a mucous surface (mouth); heart with 

 two auricles and a single ventricle; skull articulates by dou- 

 ble surfaces through ex-occipital bones ; basi-occipital carti- 

 laginous; suspensorium less complex than in Pisces; lower 

 maxillary presents a concave articular surface. 



Reptilia. Amnion and allautois present. Posterior vis- 

 ceral arches always closed, consequently no branchiae; skull 

 articulates through single surface, composed of a basi-occi- 

 pital and two ex-occipital elements; suspensorium consists 

 of a single bone (os quadratum) ; lower maxillary presents a 

 concave articular surface. 



Aves. Same as Reptilia, with the additional characters 

 union of first row of tarsal bones with lower end of tibia, and 

 second row with metatarsus; metatarsal elements anchy- 

 losed, excepting in Aptenodytes (penguin); aorta turns to 

 right; bronchial tubes open into air sacs communicating 

 generally with the interior of the bones; skin furnished with 

 feathers. 



Mammalia. Amnion and allantois present. Posterior 

 visceral arches always closed; red blood corpuscles non- 

 nucleated; diaphragm complete; skull articulates by double 

 surfaces, each of which is composed of elements from the 

 basi-occipital and ex-occipital bones; cervical vertebrae seven 

 apparent exception in Bradypus (sloth) ; lower jaw articu- 

 lates directly with skull by a convex coudyle; mammary 

 gland present; skin furnished with hair. 



Each of the above plans is distinct. No member of one 

 plan possesses a tendency to run into another. A few dis- 

 puted forms have held doubtful positions, such as 

 * Sipunculus. Really articulate (through Echiurus) in ar- 

 rangement of viscera with respect to a longitudinal axis, dis- 

 position of blood-vessel and muscular systems, and in dorsal 

 position of anus. Apparently radiate (through Holothuria) 

 in general appearance, habit, arrangement of oral tentacles, 

 tortuous alimentary canal enclosed by folds of peritoneum, 

 and structure of integument. 



Dentalium. Really molluscoid in presence of mantle, heart, 



