CHAPTER III. 



CTENOPHOKA. 



Tectonic. — The body of the Ctenophore exhibits a chief 

 axis, the poles of which are marked, one by the position of 

 the mouth, the other by the sensory organ located at the 

 apex. Perpendicular to this chief axis two mutually per- 

 pendicular secondary axes can be distinguished ; they are of 

 unequal length, and are further distinguished from each 

 other by the dissimilar organs occurring in their course. 

 The plane determined by one of these two secondary axes 

 and the chief axis we designate with Claus (No. 4) as the 

 lateral or transverse plane (Fig- 63 aa), for the tentacles are 

 situated in it, and thus a comparison with the lateral parts 

 of the body of the Bilateria is permitted. This plane is also 

 called by Chun (No. 3) the infundibular plane, since the 

 part of the gastro-canal system known as the infundibulum 

 attains its greatest dimensions in this direction. In accord- 

 ance with the comparison with the Bilateria abov^e men- 

 tioned, the plane corresponding to the other secondary axis 

 is called the sagittal plane (Fig. 63 hh), or, according to 

 Chun, on account of the extensions of the stomach occurring 

 in this direction, the gastral plane. The body of the Cteno- 

 phore is divided by these planes into four quadrants, all of 

 which, however, are not congruent with one another, as is 

 the case in quadriradial animals, but only the diagonally 

 opposite ones, each quadrant being like a reflected image of 

 the neighbouring ones. Since in radiate animals each radial 

 part (antimere) is divided into two symmetrical halves by 

 the plane of its radius, it follows that in the Ctenophora 

 each quadrant corresponds to only the half of such a radial 

 part, and that it becomes an entire antimere only after the 



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