THE EIGHT PRIMARY GROUPS OP ANIMALS. 85 



at once common to, and distinctive of, all these four groups; while 

 recent discoveries tend to widen so greatly the hiatus between 

 the Infusoria and the other three classes, that I greatly doubt if 

 the sub-kingdom PROTOZOA can be retained in its old sense. 



But if the Infusoria be excluded from it, the remaining 

 groups, notwithstanding the imperfection of our knowledge 

 regarding some of them, exhibit a considerable community of 

 partly negative and partly positive characters. 



The Spongida, Radiolaria, Ehizopoda, and Gregarinida, in 

 fact, are all devoid of any definite oral aperture ; a considerable 

 extent, and sometimes the whole, of the outer surface of the 

 body acting as an ingestive apparatus. Furthermore, the bodies 

 of these animals, or the constituent particles of the compound 

 aggregations, such as the Sponges, exhibit incessant changes of 

 form the body wall being pushed out at one point and drawn 

 in at another to such an extent, in some cases, as to give rise 

 to long lobate, or filamentous, processes, which are termed 

 " pseudopodia." 



Finally, all these classes agree in the absence of any 

 well-defined organs of reproduction, innervation, or blood 

 circulation. 



Thus, in the present state of knowledge, it seems to me that 

 the whole Animal Kingdom cannot be divided into fewer than 

 eight primary groups, no two of which are susceptible, in the 

 present state of knowledge, of being defined by characters which 

 shall be at once common and diagnostic. 



These groups are the 



VEKTEBRATA. 



MOLLUSCA. ANNULOSA. 



MOLLUSCOIDA. ANNULOIDA. 



COELENTERATA. INFUSORIA. 



PROTOZOA. 



I leave aside altogether the question of the equivalency of 

 these groups ; and, as I have already stated, I entertain some 

 doubts regarding the permanency of one the Infusoria as a 

 distinct primary division. Nor, in view of the many analogies 



