262 



CCELEN1EEATA. 



possess an cesophageal tube and a g astro-vascular canal system. Two 

 lateral tentacles, which can be retracted into pouches, are often present. 

 The Ctenophora possess a shape which can in all cases be 

 reduced to a sphere. They are radially symmetrical free-swim- 

 ming Ccelenterata of gelatinous 

 consistence. The body is often 

 bilaterally compressed, so that it is 

 possible to 

 distinguish 

 two planes 

 passing 



Gf 



FIG. 201. Cydippe, seen from the 



through the 

 long axis at 

 right angles 



to one an- I 



. to 



apical pole. S, Sagittal plane ; T, Other ; these 

 transverse plane; E, swimn ing oy.p-f-V.po,, '/ 

 plates; Gf, gastro-vascular system. c s 9 ' 



tal plane and 



the transverse plane, and are analogous 

 to the median (longitudinal vertical), and 

 lateral (longitudinal horizontal) planes of 

 bilaterally symmetrical animals (fig. 201). 

 The arrangement of the internal organs 

 bears a relation to these two planes. All 

 parts of the body which occur in pairs, as 

 the two tentacles, the gastric canals, the 

 hepatic bands of the stomach, and the 

 vessels which give origin to the eight lateral 

 canals, all lie in the transverse plane, while 

 the sagittal plane coincides with the longer 

 axis of the O3sophageal (gastric) tube (whence 

 also called the gastric plane), the two so- 

 called pol r- fields, and the terminal vessels 

 of the infundibulum. 



The infundibulum is so compressed that p IG 202. 

 its longest diameter falls in the lateral piumosa (after Chun) 



. . . . Mouth. 



plane, which on this account is sometimes 



called the infundibular plane. Since these two planes divide the body 

 into halves, which correspond with one another, and since there is no 

 division into dorsal and ventral surfaces, the arrangement of the 

 body may be said to be bi-radially symmetrical, but cannot be called 



0, 



