58 



COMPARATIVE ANATOMY. 



Fundamental Forms of the Animal Body. 



§ 49. 



Owing to the infinite variety of the external characters of animal 

 organisms, it is necessary to seek for fundamental forms to which 

 this variety may be referred. We must also ascertain the con- 

 ditions which influence and give rise to the most important modifi- 

 cations of these forms. These results may be obtained in different 

 ways. We will choose the shortest by commencing with the lowest 

 stage of the animal organism. 



This is the stage which the Gastrula form presents to us ; by its 

 wide distribution this form will provide us with the characters which 

 are best adapted for our purpose. An organism at this stage is some- 

 what spherical or oval in shape, and the mouth 

 will be found at a point on the surface. 



If we imagine an axis (Fig. 16, AB) drawn 

 straight through the digestive cavity, the pole 

 corresponding to the opening of the mouth 

 represents the oral, and the opposite the aboral 

 pole. This axis (AB) is the primary axis 

 of the body. In a body of a regular cylin- 

 drical or spheroidal shape we can imagine as 

 many lines as we please drawn through the 

 body perpendicular to this axis. (Secondary 

 axes, a b, c d.) In this instance they are all 

 equivalent. The secondary axes are in this 

 case indifferent to one another, and are cha- 

 racteristic of a lower condition. The organism, 

 either when moving freely in the water, or 

 when fixed (by the aboral pole, of course), as 

 it afterwards is, is differentiated by the develop- 

 ment of a certain number of secondary axes, 

 their development having relation to the main- 

 tenance of the balance of the body. We here, 

 then, have to do with a statical cause. The 

 development of the organism along its secon- 

 dary axes takes place through the development 

 of external appendages, tentacles and the like, 

 or through differentiation of the enteric cavity, 

 or through the laying down of other organs 

 (such as the generative glands) in the direction 

 of those axes. In consequence all the conceivable secondary axes 

 are no longer equivalent. Those along which organs are differentiated 

 are distinguished from the rest. They have passed, in fact, from the 

 previously indifferent condition to a differentiated one. The so- 

 called radiate fundamental-form of the body which is commonly 

 found in the Ccelenterata, arises in this way, as may be seen by 



Fig. 16. Diagram of 

 the axes of the body. 

 A B Primary axis, a b, 

 c d Secondary axes. The 

 lower figure is a trans- 

 verse section of the 

 upper one, showing its 

 two secondary axes. 



