146 CHORDATA. 



Subphylum D. Vertebrata (Craniata). 

 Backboned Animals. 



Chordata in which the notochord is supplemented, or more or less 

 replaced, by a mesodermic axial skeleton consisting of a skull and 

 segmented backbone. Never more than two pairs of limbs. Possess a 

 distinct head which lodges a brain that extends in front of the noto- 

 chord, two eyes with cup-shaped retina derived from the brain, two 

 auditory vesicles with semicircular canals, and two olfactory pits. There 

 is a heart with closed blood-system containing red as well as colourless 

 corpuscles, and a subordinated lymphatic (and lacteal) system com- 

 municating with a large eriteroccelic body-cavity. A spleen, and compact 

 excretory organs opening by longitudinal ducts to the exterior always 

 present. Gonads never arranged segmentally, ova usually pass into the 

 body-cavity and thence into oviducts, and sperm carried off by excretory 

 or special genital ducts. True hermaphrodites only found among fishes. 



