SUB-KINGDOM VI. VERTEBRATA. 41 



toothed tongue. Two or four plume-like gills. In front of the body a 

 muscular tube ("funnel"), through which is expelled the water which 

 has been used in respiration. An external shell in some, an internal 

 skeleton in others. Ex. Cuttle-fishes, Nautilus. 



SUB-KINGDOM VI. VERTEBRATA. 



Body composed of a number of definite segments placed one behind 

 the other in a longitudinal series. The main masses of the nervous 

 system are placed upon the dorsal aspect of the body, and are shut off 

 from the general body-cavity. The limbs (when present) are turned 

 away from that side of the body on which the main masses of the 

 nervous system are placed, and are never more than four in number. 

 In most cases a backbone or "vertebral column" is present in the 

 fully-grown animal. (Fig. 12.) 



Fig. 12. Vertebrata. Skeleton of the common Perch (Percafluviatilis). 



CLASS A. PISCES (Fishes). Breathing organs in the form of gills ; 

 heart, when present, usually of two chambers, rarely of three ; blood 

 cold ; limbs, when present, converted into fins. 



CLASS B. AMPHIBIA (Amphibians). Breathing organs of the young, 

 gills ; of the adult, lungs, either alone or associated with gills. Heart of 

 the young of two chambers, of the adult of three chambers. Blood 

 cold. Skull jointed to the backbone by two articulating surfaces 

 (" condyles "). Limbs never converted into fins. 



CLASS C. REPTILIA (Reptiles). Breathing organs in the form of 

 lungs, never in the form of gills. Heart three-chambered, rarely four- 

 chambered, the pulmonary and systemic circulations connected together, 

 either in the heart or in its immediate neighbourhood. Blood cold. 

 Skull jointed to the backbone by a single articulating surface or condyle. 



