Chap. II. MORPHOLOGY AND NOMENCLATURE. 77 



acquainted with an animal, the difficulties arising from this discrepancy between 

 natural attitudes and normal positions grow less and less. No one could be misled 

 by a description of a Turbot or a Flounder, representing their structure according 

 to their normal position, even though, in their natural attitude, they he upon one 

 side; nor would any philosophical observer describe the back of these fishes as 

 lateral on account of their natural attitude. And yet it seems hardly to have 

 occurred to some naturalists, that they make frequently a similar mistake when they 

 describe the lower animals, in almost every group, in a diflerent way ; taking every- 

 where the natural attitude, and not the normal position, as their guide. 



It is fitting, that, after alluding to the diflerent attitudes in which the Radiates 

 are found in their natural element, we should attempt to determine what is their 

 normal position. Considering the plan of their structure, we have already seen 

 that there exists in all of them an axis and centre of radiation, around which 

 all their parts are symmetrically arranged in a radiating and concentric order, even 

 though that axis or centre of structure be not the centre of figure or form. At 

 one end of this axis we invariably find the so-called mouth or adinosiome, while 

 the opposite end of the alimentary canal may have an excentric position. We 

 find, moreover, that in their natural attitude, the actinostome is in all of them 

 turned either vipward or downward, with the sole exception of the Holothurise, 

 in which it is directed forward. This exception is, moreover, of little importance, 

 since the structural relations of the Holothuria) to the other Echinoderms leave no 

 doubt as to what is their normal position ; and whatever rule Ave recognize as 

 binding for the other Echinoderms must be followed for the Holothuria3 also. 



Once agreed iipon this point, there can be no farther doubt, that, in the Eadiates, 

 the normal position of the main axis of the body is the vertical, smce all, with 

 the single exception of the Holothurite, stand in their natural element with that 

 axis in a vertical position. "We shall, therefore, not hesitate hereafter to describe 

 the Holothm-ice as if they also were in the habit of standing upright. 



It is not quite so easy to determine what should be considered as the upper, 

 and what as the lower, end of the axis. If we look to the Polyps as a guide, 

 we should certainly take the region of the actinostome as the upper end ; but 

 if we allow the relations of the higher Eadiates to influence us, we should natu- 

 rally consider the same region as the lower end of the body. It would be incorrect, 

 unquestionably, to assume that the natural tittitude of the Holothurige is to decide 

 the question, and to describe all the radiated animals with the actinostome forward, 

 as it is evident that the similarity in the natural attitude of the Holothuria3 and 

 "Worms is only an analogy, and not a leading feature applicable to the whole type 

 of Radiates. The main axis of the body of these animals is truly vertical ; and 

 in this essential relation of their whole structure to the surrounding medium, we 



