STRUCTURAL AND PHYSIOLOGICAL 



CONCHOLOGY. 



PRIMARY DIVISIONS OF THE ANIMAL KINGDOM. 



Every known animal appears to belong to one of eight principal 

 types of structnre, each of which is designated a snbkingdom. 



I. Vertebrata. The main mass of the nervons system is dorsal, 

 protected and separated from the alimentary canal by a number 

 of bony partitions, arranged consecutively in a single series and 

 known as vertebrae.* From these vertebrae proceed ventral 

 appendages termed ribs, enclosing the cavity of the body. The 

 animal is bilaterally symmetrical, in its highest development 

 provided with two pairs of limbs, anterior and posterior. The 

 sexes are distinct. Each individual is developed from a single 

 egg. Blood red, 



II. Malacozoa. The nautilus, the common garden-snail, and 

 the oyster, representatives of the three principal subdivisions 

 of the mollusca proper, are familiar examples of this subkingdom. 

 They are soft, fleshy animals, usually constructed with reference 

 to the support of an exfernal, hard, more or less enveloping 

 shell, composed of carbonate of lime. Sometimes, in the cepha- 

 lous moUusks, the shell is internal : that is, situated under the 

 dorsal skin of the animal, and in such cases it is usually more 

 simple in form, blade-like, or even reduced to a few calcareous 

 granules. The internal shell, called " pen " of the squids certainly 

 acts as a support to the animal in the same manner that the 

 vertebral column does to man, but then it is not divided into 

 segments, nor is it pierced by a spinal cord. Some cephalous 

 mollusks are entirely without vestige of shell, either external or 

 internal : in others the shell is horny instead of calcareous. The 

 nervous system in the malacozoa consists of three or four pairs 

 of ganglia, united by cords or commissures. The alimentary 

 canal is encircled below the mouth by some of these ganglia and 



* The "back-bone" is sometimes cartilaginous, and not divided into 

 vertebrae. In Amphioxus, the most degraded of the vertebrate type, the 

 circulation resembles that of the worms, the heart is wanting, the blood 

 is colorless, there ai-e no limbs : it appears to be a c mnecting link -with 

 the invertebrata— as all the succeeding subkingdoms are collectively 

 designated. 



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