76 ACALEPIIS IN GENERAL. Part I. 



and corresponding pair of nerves and vessels in either Fish, Reptile, Bird, or 

 Mammal, as a segment of the body of a Verteljrate. 



Now, it reqnires no formidable stretch of the imagination to reduce any single 

 Polyp, or any Acaleph or any Eclunoderm, to a sjiheroidal form. Indeed, the 

 sjjhere is the essential form of all Eadiates, — not the mathematical sphere, but the 

 organic sphere, loaded in different directions, according to the peculiarities of the 

 subordinate groups of this type. It has its nearest approach to the sphere in the 

 Echinus; it becomes a cylinder in the Ilolothuria; it is stellate in the Star-fish; 

 it is bell shaped in the Acaleph ; it is trumpet shaped in the Polyp ; and in all 

 it has an oral opening in the centre of structure, which nuiy not lie the centre 

 of figure. 



Keei^ing in mind this starting point, if we consider the natural position of the 

 animal in its element, we find in Polyps the so-called mouth turned upward in 

 the centre of the Ijroadest exjiansion of one side of that organic and flexilile sphere, 

 while the ojtposite end, more or less tapering, becomes a base of attachment. 

 Hydroids retain the same attitude, and bear the same general relation to the 

 surrounding medium. Not so with the Medusa', in which the sphere is freed from 

 all attachment and the oral aperture turned downward, the whole body being 

 more or less hemisjiheric or bell shaped. In Echiuoderms we have not only the 

 Crinoids, recalling, in their relations to the surrounding mediums, the Polyps and 

 Hydroids, but also the Sea-urchins and Star-fishes, in which the mouth is turned 

 downward as in Medusa^, and the Ilolothurians, in which it is directed forward. 

 In order, therefore, to have a normal position for all Radiates, we must compare 

 them with one another, not in their natural attitudes, but in such a jiosition as 

 would exhibit, in all, the centre of their structure in the same relation to the 

 surrounding medium. 



The necessity of thus distinguishing the natural attitude and the normal position 

 of animals is particularly obvious in the study of Radiates. But the distinction 

 is quite as important in other branches of the animal kingdom. Everywhere the 

 possibility of acquiring an insight into the typical structure of any natural group 

 depends primarily upon the position in which its representatives are compared. 

 Had not Rathke taken these relations into consideration, we should not know the 

 antagonism which prevails between the Articidates and MoUusks in their embry- 

 onic development. Without keeping tliem in mind, we shall never l)e able to 

 homologize the Bryozoa and Tunicata with the other Acephala. Without knowing 

 that an animal may move in a position entirely at variance with the normal position 

 of the other representatives of its class, a description of its characteristic features 

 may appear in direct contradiction to its habits, or mislead us with reference to 

 its natural relation to the surrounding medium. In proportion as we are better 



