8 ANATOMY OF VERTEBRATES. 



parial fins, supported by rays ; endoskeleton in most, more or 

 less ossified ; hyoid arch attached to tympanic pedicle ; scapular 

 arch attached to the occiput ; no sternum : skin defended by scales 

 or plates ; brain with predominant mesencephalon ; myelon opake, 

 inelastic; a sympathetic nerve; organ of smell double ; eyes usually 

 large, with bony sclerotic ; auditory labyrinth with three semi- 

 circular canals, in the cranial cavity ; mouth formed by upper and 

 lower jaws, opening at the fore part of the head, and admitting the 

 respiratory currents ; intestine, in most, with pyloric appendages 

 and spleen ; anus in front of urethra ; air-bladder in most ; gills, 

 free ; branchial outlet single on each side, defended by a bran- 

 chiostegal flap, with one or more rays ; testes (' milt ') and ovaries 

 (' roe ') large, with continuous ducts in most ; ova very numerous 

 and small, simultaneously developed, and impregnated, usually, 

 externally ; no amnios or external allantois. 



Subclass III. Plagiostomi. — Endoskeleton cartilaginous, or 

 partially ossified; scapular arch detached from occiput; exo- 

 skeleton as osseous granules or tubercles ; body with medial and 

 parial fins, the hinder pair pelvic in position; caudal-fin with 

 produced upper lobe ; brain with the prosencephalon predominant ; 

 auditory labyrinth in a special chamber ; mouth, in most, a wide 

 transverse slit, opening below the head ; intestine with a spiral 

 valve, pancreas, and spleen ; no air-bladder ; bulbus arteriosus 

 with numerous rows of valves; gills, in most, fixed, and with 

 several branchial outlets on each side ; testes of moderate size, 

 with sperm-duct and copulatory apparatus ; ovaries with few and 

 large ova, successively developed and conveyed away by a 

 detached oviduct ; ova impregnated and, in some, developed in- 

 ternally ; embryo without amnios or allantois, and with deciduous 

 external gills. 



Subclass IV. Dipnoa. — Endoskeleton more or less ossified ; 

 ribs wanting, or short and free ; parial members as legs ; brain with 

 predominant prosencephalon ; optic nerves not decussating ; audi- 

 tory labyrinth in a special chamber, but with only the ' fenestra 

 vestibuli ; ' nostrils communicating with the mouth ; intestine, 

 with pancreas and spleen ; air-bladder as a pair of lungs, com- 

 municating by a duct and glottis with the haemal side of the 

 pharynx; heart, in most, with one ventricle and two auricles. 

 Testes of moderate size, with sperm-ducts, but no intromittent 

 organs or claspers ; ovaries with detached oviducts ; ova simulta- 

 neously developed, and, in most, impregnated externally. Embryo 

 without amnios or allantois, and with external gills. 



