106 RELATIONS OF MAN WITH EXTERNAL NATURE. 



Cumerian Arrangement. 



VERTEBRATA 



MOLLUSCA .... ARTICULAT A 



KADIATA 



Modern Arrangement. 



VERTEBRATA 

 MOLLUSCA ANNULOSA 



MOLLUSCOIDA . ANNULOIDA 



"-... CCELENTERATA . 



PROTOZOA 



General Characters of the Subkingdoms. 



Vertebrata. The vertebrate animals are distinguished by the possession of 

 an internal skeleton, the central part of which, the back-bone, forms a longi- 

 tudinal bony or cartilaginous axis, usually divided into segments called verte- 

 6rce, the entire axis being named the vertebral column ; anteriorly, this internal 

 skeleton is expanded into the cranium; laterally, the vertebral column fre- 

 quently presents symmetrical pieces, named ribs; and, besides these, there 

 may exist two or four, but never a greater number, of larger lateral append- 

 ages, placed symmetrically on each side, and named limbs. Within the verte- 

 bral column, and (supposing this to be placed horizontally), above the more 

 solid part formed by the so-called bodies of the vertebra, is an elongated cav- 

 ity continuous in front with the cranial cavity, and named the neural cavity, 

 because it contains the great nervous axis composed of the brain and spinal 

 cord; whilst below the vertebral column, and inclosed within the ribs, when 

 these exist, is a larger cavity, called the hcemal cavity, because it lodges the chief 

 organs of the circulation, the heart and great bloodvessels ; but besides these, 

 it also contains the digestive, absorbent, respiratory, and reproductive organs, 

 as well as the cords and ganglia of the sympathetic nerves ; it forms therefore a 

 distinct perivisceral cavity. 



The nervous system is more highly developed than in the other subking- 

 doms ; not only are the senses more perfect, but the brain or cerebral portion 

 6f the nervous system is highly developed, and exhibits a capacity ta be asso- 

 ciated with superior mental endowments. The jaws are formed by modified 

 parts of the head, have no true analogy to limbs, and move vertically ; teeth 

 are often present, composed of dentine or modified bone covered with other 

 hard material formed from the mucous membrane of the mouth ; they consti- 

 tute parts of the external skeleton. The alimentary canal, besides possessing 

 a distinct stomach, salivary glands, and liver, is also provided with a pancreas. 

 Special absorbent or lymphatic vessels exist, ending in a true blood-system of 

 vessels, and containing a fluid in which colorless corpuscles are present ; the 

 absorbents proceeding from the alimentary canal convey chyle, and are named 

 lacteals. The circulating system is highly developed, consisting of a haart 

 divided into two, three, or four cavities, connected with distinct arteries, capil- 

 laries, and veins, and containing blood provided with both white and red cor- 

 puscles : a portal system of veins, transmitting blood from the alimentary 

 canal to the liver, is also peculiar to the Vertebrata. In the lowest vertebrate 

 animal known, viz., the Amphioxus or lancelet, an exception is found in regard 

 to the blood and the heart, the blood containing only colorless corpuscles, and, 

 in place of a single heart, numerous contractile sacs are found in the course of 

 the chief bloodvessels. One at least of the so-called vascular glands, viz., 



