376 HOMOLOGIES OF THE RADIATA. Part V. 



opposite to one another, while the back of a Starfish corresponds to the posterior 

 extremity of an Holothuria. To bring, therefore, all the Radiates into a uniform 

 normal position, we must place them in that attitude of their main axis, which will 

 indicate prominently their peculiarity as a primary division of the animal kingdom ; 

 and that attitude is the vertical, as it is, also, the natural attitude of a large 

 majority of them. 



To facilitate our generalizations, we may well assume that all the Radiates are 

 spheroidal. Those that have not really that form, may readily be reduced to it, 

 by slight changes of their different diameters, and without altering any of the 

 primary relations of the plan of their structure. 



The essential elements of the structure of these spheroidal bodies are spherical 

 wedges, arranged symmetrically around a vertical axis. Of course, we have not 

 to deal here with mathematical figures, but with tlie elements of a living sphere, 

 loaded in every direction with those structural differentiations which determine the 

 jjeculiarities of organic structures. In consequence of this unequal Aveight of the 

 different diameters of the body, we find that the opposite poles of our organic 

 sphere are provided with parts of a different nature, and perform different functions. 

 The sides also present similar differences, in consequence of the unequal develop- 

 ment of alternate zones, extending from pole to jiole, and of similar inequalities 

 along the same zone. The so-called mouth is always placed at one of these poles, 

 and from it radiate the most prominent organs, in consequence of which I have 

 called this side of the body the oral, or acfinul area, and the opposite side the 

 aboral, or ahadinal area. This mode of designating these regions applies in every 

 case, and we thus get rid of the difficulty ai-ising from the inverse position of 

 many of these animals. The zones, extending from pole to pole, differ chiefly in 

 the differentiation of the substance, and the iDOsition of different systems of organs 

 alternating with one another at the periphery of the body. Thus, in Sea-urchins, 

 we have the ambulacral system alternating with the genital organs, Avhile the 

 digestive cavity occupies the centre; in Polyps, the radiating partitions to which 

 the genital organs are attached, alternate likewise with the radiating chambers 

 leading into the tentacles. For this reason I have adopted the names of ambu- 

 lacral and interambulacral zones, to designate the alternating structural regions prom- 

 inent upon the surface of all the Radiates. I have selected these names, not 

 because they are the most apjoropriate, but because they recall the familiar 

 structure of the Echinoderms, and may facilitate the comparisons between the 

 different classes of these animals. The differences in the structure of one and the 

 same zone, may best be determined with reference to the actinal and abactinal pole. 



