XX INTRODUCTION. 



they differ in their more elaborate internal organization. The 

 Cceleirnintha are more perfect forms of the Parenchymatous 

 Entozoa. The Rotifera, formerly confounded with the Infu- 

 soria, exhibit manifest analogies with the articulated Crusta- 

 ceans, as in fact do the Epizoa. The Echinodermata alone 

 appear to form an isolated group, properly belonging to the 

 division under consideration. 



No animal substance yielding a medicinal agent is ob- 

 tained from this division of the animal world. 



III. Division. HOMO GANGLI ATA.* 



The Articulated division of the animal world is character- 

 ized by a nervous system, much superior in development to 

 that possessed by the two preceding, indicated by the superior 

 proportionate size which the ganglion ic centres bear to the 

 nerves which emanate from them. The presence of these 

 central masses of neurine admits of the possession of external 

 senses of a higher class than might be expected among the 

 Acrita or Nematoneura, and gives rise to a concentration of 

 nervous power, which allows of the existence of external 

 limbs of various kinds, and of a complex muscular system, 

 capable of great energy and power of action. 



The nervous centres are arranged in two parallel lines along 

 the whole length of the body, forming a series of double gan- 

 glia or brains belonging apparently to the individual seg- 

 ments of which the animal is composed. The anterior pair, 

 placed invariably in the head above the oesophagus, and con- 

 sequently upon the dorsal aspect of the body, seems more 

 immediately appropriated to the higher senses, supplying 

 nerves to the antennaB or more special instruments of touch, 

 to the eyes, which now manifest much complexity of struc- 

 ture, to the auditory apparatus where such exists, and proba- 

 bly to the senses of taste and smell. This dorsal or anterior 

 pair of ganglia, which evidently is in relation with the higher 

 functions of the economy of the creature, is brought into 

 communication with the series of nervous centres placed 

 along the ventral aspect, by means of filaments which em- 

 brace the oesophagus, and joins the anterior pair placed be- 

 neath it ; the whole system may therefore be regarded as a 

 series of independent brains destined to animate the seg- 

 ments of the body in which they are individually placed. 

 Such a multiplication of the central organs of the nervous 





