ANATOMY OF VERTEBRATES. 7 



Thus vertebrates might he binarily divided into oviparous, I. 

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

 allantoic or abranchiate ; into Ucematothcrmal, 1 having the four- 

 chambered heart, spongy lungs, hot blood, and Hcematocryal, 2 

 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 

 perished, and combining with paheontological 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 being called 

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

 * sub-class.' 



I. Dermopteri. III. Plagiostomi. 



II. Teleostomi. IV. Dipnoa. 



V. MONOPNOA. 



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 cascal 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 Arnmo- 

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



Subclass II. Teleostomi. 4 — Body pisciform, with medial and 



1 Gr. haima, blood ; thermos, hot. 



2 Gr. haima, blood; cruos, cold. 



3 Retaining the notochord or primitive basis of the vertebral column. 



4 This word (from Gr. telos, end or completion ; stoma, 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. 



