Chap. II. MORPHOLOGY AND NOMENCLATURE. 75 



diversified parts of the same system. Surel}', tlie head of a fish is not to be called 

 by aiiother name than that of Birds or Mammals, because it is not sepai\ited from 

 the chest by a long neck ; nor are we to have as many different names as there 

 are different condjinations of structure in the j)firt>s of the face. The olfactory 

 organ, or the nose, must be called nose or olfiictory organ, whether it be as promi- 

 nent as the proboscis of the elejjhant, or as blunt as the snout of a fish. The 

 ear must be called ear, be it ever so prolonged externally, or entirely concealed 

 below the surface of the head. All this can be readily done among Vertebrates 

 and among Articulates, because the structure of all these animals is sufficiently 

 well knoAvn to force a uniform nomenclature upon the attention of any one who 

 studies them. 



The correspondence of the rings of an Articulate, be it a Worm or a Crus- 

 tacean or an Insect, is evident, whether it be altogether deprived of locomotive 

 appendages, or provided with legs only, or Avith wings as well as legs ; and it 

 will be at once understood, by any one who extends these coiuparisons sufficiently, 

 that the parts now generally called legs and wings among Insects, though bearing 

 the same names for the present, are not homologous with legs and wings in 

 Vertebrates. The parts of the mouth of a sucking or a chewing Insect, on the 

 contrary, will with the same readiness l)e recognized as homologous with their 

 so-called legs. Unfortunately, this is not the case with the Kadiates. Wc find 

 almost as many different opinions respecting the parts of Echiuoderms, Acalephs, 

 and Polyps, as there are writers on the subject. Even with reference to Echi- 

 uoderms alone, there arc authors who have denied the homology of the solid parts 

 of the Sea-urchins with those of the Star-fishes, and described the solid frame of 

 the one as external, and that of the other as internal. 



It is not my intention here to consider the general homologies of the Kadi- 

 ates in detail, as I shall take up the subject again at the end of this monograjoh. 

 But for the piu'pose of introducing a more uniform nomenclature among these 

 animals, or, at least, paving the way to it, I will attempt such a general com- 

 parison between them as may fficilitate a reference of the parts of one class to 

 the parts of another. 



The plan upon which the Radiates are Imilt is so peculiar, and so distinct from 

 that of the Mollusks, Articulates, and Verteln-ates, that the essential elements of 

 their structure are entirely different. A common Star-fish or a common Sea-urchin 

 is as readily divided into five segments, as a common Medusa into four, or an 

 individual animal of a Gorgonia into eight, or that of an Actinoid Polyp into a 

 larger number, according to different families. Such segments bear to the body 

 as a whole, a relation similar to that observed in the ring of an Insect as one 

 of the essential elements of its structure, or a vertebra with its muscular band 



