8-1 INTRODUCTION TO CLASSIFICATION. 



cilia which line the general cavity of the body can be regarded 

 as such ; and a nervous system has, at present, been clearly made 

 out only in the Ctenophora. Here its central mass occupies a 

 position which is very unlike that in which the principal masses 

 of the central nervous system are found in other invertebrate 

 animals, being situated upon that side of the body which is dia- 

 metrically opposed to the mouth. 



Whatever extension our knowledge of the nervous apparatus 

 of the Crelenterates may, and not improbably will, receive from 

 future investigators, the positive characters afforded by the histo- 

 logical features of their substance, and the free opening of their 

 alimentary canal into the general cavity of the body, are such 

 as to separate them, as a sub-kingdom, as sharply defined and 

 devoid of transitional forms as that of the Vertebrata, from the 

 rest of the Animal Kingdom. 



Great difficulties stand in the way of any satisfactory group- 

 ing of the remaining classes, if we are determined to remain 

 true to the principle that the definition of a group shall hold 

 good of all members of that group, and not of any others, 

 a principle which lies at the foundation of all sound classification. 



In possessing cilia, as locomotive and ingestive organs ; in 

 being provided with a contractile water receptacle with canals 

 proceeding from it (in some cases at any rate) into the substance 

 of the body ; in their tendency to become encysted and assume 

 a resting condition, the INFUSORIA undoubtedly exhibit analogies 

 with the lower Annuloida, such as the Turbellaria, Rotifera, and 

 Trematoda. 



But the entire absence, so far as our present knowledge 

 goes, of a nervous system, the abrupt termination of the gullet 

 in a central semi-fluid sarcodic mass, and the very peculiar 

 characters of the reproductive organs, of the Infusoria, separate 

 them widely from the Annuloida, though it seems to me not 

 improbable that the gap may hereafter be considerably dimi- 

 nished by observation of the lower forms of Turbellaria. 



At present the ^Infusoria are usually regarded as forming part 

 of the same sub-kingdom as the Spongida, Radiolaria, Khizopoda, 

 and Gregarinida, and as closely allied to them. But, so far as I 

 am aware no definition can be framed which will yield characters 



