HOMOLOGIES OF THE RADIATA. 



SECTION I, 



GENERAL HOMOLOGIES. 



In order to compare the different systems of organs in animals whose natural 

 attitudes in the surrounding elements may be extremely diversified, we must first 

 bring them all into the same position ; or, in other words, we must discriminate 

 between their natural attitude and their normal position. No branch of the animal 

 kingdom exhibits so great a diversity of attitudes as the Radiates. Some of them 

 are always found mouth upwards, others mouth downwards, or lying upon one or 

 the other side ; and before they have been placed in a corresponding position, no 

 accurate comparison between them can be instituted. It is, in my opinion, a 

 mistake to place them, for such a purpose, in the position in which we are accus- 

 tomed to describe animals of other branches. The very plan of their structure, 

 characterized by radiation, forbids this. The main axis of their body is not a 

 longitudinal axis, as in Vertebrates, but a vertical axis, around which the jjrimary 

 elements of their structure are symmetrically arranged. Most of them, moreover, 

 assume in nature an attitude corresponding to this view of the subject. An attempt 

 to place a Polyp, or a Jelly-fish, or a common Echinus on one side, Avitli the mouth 

 forward, does not modify the plan of their structure, and bring it in any way 

 nearer to that of bilateral animals, with a distinct anterior and posterior end, an 

 upper and a lower side, a right and a left. In whatever position a Radiate may 

 be found, its structural elements retain their radiating arrangement around the main 

 axis, and taking the bulk of the representatives of this type as our guide, that axis 

 must be considered as a vertical axis. It remains so even in those Radiates which, 

 like the Holothurians, move movith forward, i-esting uj)on one side ; for that side 

 bears the same primary relations to the main axis, as in those which move or stand 

 mouth upward or downward. The so-called dorsal or ventral side of an Holo- 

 thuria, a Sjoatangus, or a Starfish, are neither homologous among themselves, nor 

 do they correspond to the back or lower side of any Vertebrate, or Articulate, 

 or Mollusk. Holothuria and Spatangus rest upon sides which are homologically 



