398 PHILOSOPHY OF ZOOLOGY. 



II. 



INVERTEBRATA. 



Invei^tehral Animals. 



Characters.-— Animals destitute of a skull and ver- 

 tebral COLUMN, FOE THE PROTECTION OF THE BRAIN 

 AND SPINAL MARROW. 



The invertebral animals liave few characters of a posi- 

 tire kind, which they possess in common. The skin 

 consists only of a corium and cuticle, both of which, ac- 

 cording to circumstances, are furnished with appendices, in 

 the form of shells, crusts, scales, or hairs. These gene- 

 rally supply the place of the osseous system, serving as a 

 protection to the viscera, and as supports to the muscles. 

 The blood, in those cases where a circulating fluid can be 

 detected, is usually of a white or grey colour, seldom in- 

 clining to red. When there are both systemic and pulmo- 

 nic ventricles, they are not united, as in the vertebral ani- 

 mals. With the exception of the genuine viviparous mode 

 of reproduction, the invertebral animals exhibit all the 

 other modifications of that function. 



In attempting the division of invertebral animals into 

 subordinate groups, the condition of the nervous system 

 furnishes characters of importance. In one extensive class, 

 which, from the starry form of the species, has been termed 

 Radiata, the nervous matter appears to be disseminated 

 among the different organs, and never appears in the form 

 of a brain, with its connected filaments. In another class, 

 equally extensive, and which, with propriety, may be deno- 

 minated Gangliata, the brain appears in the form of a 

 collar, surrounding the gullet, near its entrance into the sto- 

 mach, and sending out filaments, which, in their course, ex- 

 pand into ganglia. 



