BILATERAL 8YMMETET. 



27 



divides the antimeres from each other. The first, in which the 

 greater number of organs are repeated, may be designated the 

 transverse plane, while the second, corresponding to the median plane 

 of bilateral animals, is known as the sagittal plane (fig. 146). 



FIG. 14a. Acalepha larva (Ephyra). 

 Bk, marginal body ; Gf t gastric fila- 

 ment. Re, radial-canal ; O, mouth. 



FIG. 145. Ctenopheran seen from 

 above. S, sagittal plane; T, trans 

 verse plane ; R, vibratile plates ; Gf, 

 gastric canals. 



In the bilateral arrangement, which is found also in each individual 

 antimere of the Radiata, only one plane, the median plane, can be 

 imagined, which passes through the chief axis and divides the body 

 into two exactly similar parts (right and left). These two halves, as 

 opposed to antimeres, - may be termed parameres. 

 In bilateral animals we distinguish an anterior and 

 posterior end, a right and a left side, and a dorsal 

 and a ventral surface. The unpaired organs are 

 placed in the middle line, on each side of which, in 

 the two halves of the body, are placed the paired 

 organs. The plane which is placed at right angles 

 to the median plane (passing from right to left) and 

 separates the unlike dorsal and ventral halves of 

 the body, is known as the lateral plane. The anti- 

 meres of the Radiata also consist of two parameres, 

 and are therefore bilateral, in that the vertical plane 

 passing through the radius like the median plane 

 divides them into two similar parts. 



The same groups of organs or similar parts of 

 the same organ may also be repeated in a longitu- Fro. is. Segmented 

 dinal direction. This occurs especially frequently JS^VwSj!* 

 in bilateral, less frequently in radiate animals mentary canal; c, 



/ j 7 v \ mi -U j .LI -I j_ -L- cirrus: jF, tentacle 



(strooita). Ine body thus obtains a segmentation, 



and is divisible into successive sections, the segments or metameres, 



