INTRODUCTION. XIX 



IV. Division. NEMATONEURA. 



In the second division of the Radiata of Cuvier, the nervous 

 matter is distinctly aggregated into filaments, and in some 

 cases nuclei of neurine, which may be regarded as rudimen- 

 tary nervous centres, have been noticed. It is to be lament- 

 ed, however, that in this most interesting group of animals, in 

 which is the first development of most of the organs subser- 

 vient to the vital functions, the extreme minuteness of some 

 genera, and the difficulty of distinctly observing the nervous 

 system in the*larger species, have prevented our knowledge 

 regarding their organization, in this particular, from being of 

 that satisfactory character to which it may hereafter attain. 



Owing to the want or imperfect condition of the nervous 

 centres, the Nematoneura are necessarily incapable of possess- 

 ing external organs of the higher senses, the general sense of 

 touch being as yet the only one of which they are indubitably 

 possessed. Yet in their muscular system they are much more 

 efficiently provided than the Acrite orders, as the development 

 of nervous threads of communication renders an association 

 of muscular actions possible ; and therefore, co-apparent with 

 nervous filaments, are distinguished in the structure of the 

 Nematoneura distinct fasciculi of muscular fibre, and powers 

 of locomotion of a much more perfect description. 



The digestive apparatus is no longer composed of canals 

 merely excavated in the parenchyma of the body, but is pro- 

 vided with distinct muscular and membranous walls, and 

 loosely attached in an abdominal cavity. 



The circulation of the nutritious fluid is likewise carried 

 on in a separate system of vessels, distinct from the alimen- 

 tary apparatus, yet still unprovided with a heart or exhibiting 

 pulsations for the forcible impulsion of the contained blood. 



The fissiparous mode of reproduction is no longer wit- 

 nessed, an obvious consequence of the increased complexity 

 of structure, and these animals are for the most part androgy- 

 nous, or capable of producing fertile ova, without the coopera- 

 tion of two individuals. 



It will be perceived, that this division, however well sepa- 

 rated from the preceding by physiological characters, is, in a 

 zoological point of view, principally composed of groups de- 

 tached from the members of other orders. The Bryozoa are 

 evidently dismemberments of the family of Polyps, from which 



