CTENOPHORA. 



263 



bilaterally symmetrical, although each half possesses this property. 

 The body is divided by these two perpendicular planes into four 

 similar quadrants. 



Locomotion is principally effected by the regular vibration of the 

 hyaline swimming plates, which are disposed over the surface of the 

 body in eight meridional rows, in such a way that each quadrant 

 possesses two rows of plates, a transverse and a sagittal (fig. 202). 

 Locomotion is also assisted by the contractility of the muscle fibre* 

 of the gelatinous tissue ; this contractility in the band-shaped 

 Cestidce causes an undulating motion of the whole body. 



The mouth, which is sometimes surrounded by umbrella-shaped 

 lobed processes of the gelatinous tissue, leads into a wide (Beroe) or 

 narrow oesophageal tube, which in the latter case soon becomes 

 flattened and broad. The cesophageal tube is furnished with two 

 hepatic bands, and com- 

 municates posteriorly, 

 by an opening capa- 

 ble of being closed by 

 muscles, with the gastric 

 cavity, or, as it is com- 

 monly called, the in- 

 fundibulum. The long 

 cesophageal tube projects 

 and opens freely into 

 the infundibulum, and 

 is completely surrounded 

 by the gelatinous sub- 

 stance, as far as the level 

 of the two longitudinal vessels which accompany the two lateral 

 surface-; in the transverse plane. 



The infundibulum, which is in all cases compressed in a direction 

 at right angles to the oesophageal tube, gives oft' eight vessels to 

 the swimming-plates. These vessels have a bi-radial symmetry. It 

 also gives off two vessels, which are dilated into two terminal sacs ; 

 the latter surround the sense-organ at the aboral pole, which is 

 known as the.otolith vesicle, and each of them opens to the exterior 

 by an orifice which is placed in a diagonal plane and is capable of 

 being closed. Two tentacular vessels may arise from the bottom of 

 the infundibulum. The internal surface both of the resophageal tube 

 and of the infundibulum and its vessels seem to be completely clothed 

 with cilia. 



FIG. 203. Aboral end of Callitmirn lialata (after E. 

 Hei'twig) . a 1 , The two polar spaces ; , the beginning 

 of the eight rows of swimming plates, between which 

 the otolith vesicle and the nerve plate are seen. 



