ANATOMY OF VERTEBRATES. 7 



Thus vertebrates might be binarily divided into oviparous, I. 

 II. in., and viviparous, iv. ; into aiiallantoic or branchiate and 

 allantoic or abranchiate ; into H&matothermal, 1 having the four- 

 chambered heart, spongy lungs, hot blood, and Hcematocryal? 

 having less perfect breathing organs, less complex heart, with cold 

 blood ; and each of such divisions are artificial and convenient. 

 It suits my present purpose to adopt the latter. 



8. Subclasses of Hcematocrya. - With the best insight 

 peering into the dark vistas of the remote past - - that one can 

 command into the nature of the strange forms which then 



o 



perished, and combining with pakeontological research the results 

 of anatomical and developmental scrutiny of existing vertebrates, 

 the following seem to be the best defined cold-blooded groups, 

 each with such characters in common as leads to their beino; called 



O 



' natural,' and of a value which may be expressed by the term 

 6 sub-class.' 



I. DERMOPTERI. III. PLAGIOSTOMI. 



II. TELEOSTOMI. IV. DIPNOA. 



V. MONOFNOA. 



Subclass I. DERMOPTERI. Body vermiform, limbless; endo- 

 skeleton membrano- cartilaginous and notochordal, 3 ribless ; skin 

 scaleless, lubricous ; a vertical fin-fold bordering the hind part of 

 the body, without fin-rays ; myelon opaline, ductile, elastic ; no 

 sympathetic nerve ; organ of smell single ; eyes wanting, or very 

 small ; "optic nerves not crossing each other ; auditory labyrinth 

 of one or two semicircular canals ; mouth jawless, or suctorial ; 

 alimentary canal straight, simple, without crecal appendages, 

 pancreas, or spleen. Branchial function independent of the mouth ; 

 heart, without ' bulbus arteriosus ; ' a pulsatile portal sinus ; no 

 swim-bladder ; testes and ovaria elongated plates without ducts ; 

 generative outlet peritoneal ; ova numerous, small, simultaneously 

 developed, and impregnated externally ; cleavage of yolk entire ; 

 no amnios or allantois ; a metamorphosis, as, e. g. from Ammo- 

 ccetes to Petromyzon, after the third year from the egg. 



Subclass II. TELEOSTOMI. 4 - -Body pisciform, with medial and 



1 Gr. /Kiima, blood ; thermos, hot. 



2 Gr. haima, blood; cruos, cold. 



Retaining the notochord or primitive basis of the vertebral column. 



This word (from Gr. telos, end or completion ; fifonui, mouth ;) refers to the com- 

 pletion of the mouth by opposing upper and lower jaws, and also to its terminal 

 position, opening at the fore end of the head. 



