THE LOWER FORMS OF LIFE. 69 



PART III. 



THE CCELENTERATA. 



CHAPTER I. 

 THE HYDROZOA. 



CUVIER included the Protozoa and the group now to be considered 

 in the sub-kingdom Radiata, so called because the animals were 

 formed upon a type which consisted of a centre and parts radiating 

 therefrom. Agassiz, in his recent Graham lectures, has made the 

 most of this by endeavouring to show that the great animal king- 

 dom was specially created under four types, of which the radiate, 

 as formed by Ouvier, was the lowest. But then, to support his 

 view, he has been obliged to take his best specimens of the radiate 

 type from the Echinodermata, an order placed by all scientific 

 writers of the present day in a much higher position in the scale of 

 being. The two things cannot be reconciled. We must either 

 group together animals having a similar external arrangement of 

 organs, but with a totally .different internal anatomy, or we must 

 do that which has been done viz., bring together great classes 

 which have a similarity of organisation, and place them in the scale 

 as they rise one above the other in the links of a great chain, each 

 larger than that which precedes it. It is a favourite argument that 

 such a chain does not exist in nature. This is, literally speaking, 

 true ; but the difficulty is at once got over by dividing the chain 

 into six parts, each link in each part being of greater value than its 

 predecessor, and the same with each chain when complete. Instead 

 of four we thus make six great types of organised life, one of plants 

 and five of animals. I have already considered two, and I now pro- 

 ceed to point out the salient parts of the third. I propose, however, 

 to sink altogether the term Eadiata, and to adopt for it that of Coelenr 

 terata a plan pursued by the best modern comparative anatomists. 

 The name is at once suggestive to the scholar of that all-important 

 organ, the stomach (koilia, from koilos hollow) ; and it owes its 

 origin to the fact that in the group of animals to which it belongs 

 a stomach is for the first time differentiated, as it is called in scien- 

 tific language. 



