216 ANNELIDA. 



depend ; and by following out this great principle, we shall be easily 

 able to account for the progressive steps whereby the Articulata become 

 more and more perfectly organized, as we trace them in the series above 

 indicated. In proportion as we have found the segments of the body 

 to become less numerous, the appended limbs stronger, the outward 

 skeleton more dense, and the muscular powers more energetic, we shall 

 find the abdominal ganglia to diminish in number by becoming consoli- 

 dated into larger masses, increasing in size and energy in accordance 

 with the development of the limbs over which they preside ; and, in the 

 same manner, we shall observe the senses assume greater perfection of 

 structure, and the instincts become more developed, as we find the 

 cephalic or anterior pair of brains increasing in proportionate bulk. 



(564.) Among the Homogangliata are likewise to be detected the 

 first traces of the sympathetic or splanchnic nervous system. This con- 

 sists of delicate filaments which are distributed upon the alimentary 

 canal, presenting, in their course, ganglionic enlargements, and anasto- 

 mosing, some with the cesophageal ring, and others with the cerebral or 

 encephalic ganglia*. 



(565.) These observations will suffice to introduce the student to the 

 Homogangliate division of the animal world, and to direct his attention 

 to those physiological points connected with the nature of their nervous 

 system which will be more fully laid before him in the following pages. 



CHAPTER X. 



ANNELIDA. 



(566.) THE lowest class of articulated animals comprehends an exten- 

 sive series of creatures generally grouped together under the common 

 name of Worms. In the outward form of their bodies many of them 

 resemble some of the more perfect Entozoa, and we need not therefore 

 be surprised that, in ordinary language, they are frequently confounded 

 together. But whatever may be the similarity in outward appearance 

 between the more perfect intestinal worms and the animals belonging 

 to the class upon the consideration of which we are now entering, the 

 examination of their anatomical structure will at once show that they 

 differ widely from each other, and have thus been properly separated 

 by a considerable interval in all the more modern systems of zoological 

 arrangement. 



(567.) The principal characters which serve to distinguish the 

 ANNELIDA from other forms of the animal world are readily appreciated, 



* Vide Brandt, Bemerkungen iiber die Mundmagen- oder Eingeweidenerven der 

 Evertebraten. Leipzig, 1835. 



