VERTEBRATA: GENERAL CHARACTERS. 447 



may be partially undeveloped ; but there are never more than 

 two pairs, and they always have an internal skeleton for the 

 attachment of the muscles of the limb. 



A specialised blood-vascular or " haemal " system is present 

 in all the Vertebrata ; and in all except one the Amphioxus 

 there is a contractile cavity or heart, which never consists of 

 less than two chambers provided with valvular apertures. In 

 all the Vertebrata the heart is essentially a respiratory heart 

 that is to say, it is concerned with driving the impure or venous 

 blood to the breathing organs; and in its simplest form (fishes) 

 it is nothing more than this. In the higher Vertebrates, how- 

 ever, there is superadded to this a pair of cavities which are 

 concerned in driving the pure or arterial blood to the body. 

 In the case of the Mammals, these two circulations are often 

 spoken of as the "lesser" or "pulmonary" circulation, and 

 the " greater " or " systemic " circulation. 



In all Vertebrates there is that peculiar modification of the 

 venous system which is known as the "hepatic portal system." 

 That is to say, a portion of the blood which is sent to the ali- 

 mentary canal, instead of returning to the heart by the ordinary 

 veins, is carried to the liver by a special vessel the vena 

 portcz which ramifies through this organ after the manner of 

 an artery. 



In all Vertebrates, also, is found the peculiar system of ves- 

 sels known as the "lacteal system." This is to be regarded 

 as an appendage of the venous system of blood vessels, and 

 consists of a series of vessels which take up the products of 

 digestion from the alimentary canal, elaborate them, and finally 

 empty their contents into the veins. 



Lastly, the masticatory organs of Vertebrates are modified 

 portions of the walls of the head, and never " hard productions 

 of the alimentary mucous membrane, or modified limbs " (Hux- 

 ley), as they are amongst the Invertebrata. 



The above are the leading characters of the Vertebrata as a 

 whole ; but before going on to consider the primary divisions 

 of the sub-kingdom, it may be as well to give a very brief and 

 general description of the anatomy of the higher and more 

 typical Vertebrates, commencing with their bony framework, 

 or skeleton. 



The skeleton of the Vertebrata may be regarded as consisting 

 essentially of the bones which go to form the head and trunk 

 on the one hand (sometimes called the " axial " skeleton), and 

 of those which form the supports for the limbs (" appendicular " 

 skeleton) on the other hand. The bones of the head and 

 trunk may be looked upon as essentially composed of a series 



