THE CTENOPHORA. 121 



ops is), others being ribbon-like, as Cestum, the Venus' girdle, 

 others sac-like, as Beroe and Idyia. Indications of a radial 

 symmetry are seen in the eight longitudinal bauds of cilia- 

 plates (cp) which serve as locomotor organs, but this is thrown 

 into the background by the more pronounced bilaterality. 

 The stomodaeum is flattened in one plane and the gastric 

 cavity (g) in a plane at right angles to this, two tentacles, one 

 on each side of the body, lying also in this plane. It is possi- 

 ble accordingly to recognize a sagittal plane, that of the 

 stomodseurn, and a transverse plane, that of the gastric cavity, 

 and corresponding axes. 



The mouth lies at the extremity of the vertical axis which 

 is directed backwards in locomotion, and opens into the ecto- 

 dermal stomodseum, which is flattened parallel to the sagittal 

 plane. At its upper end the stomodseum opens into the en- 

 dodermal gastric cavity (g) or so-called "funnel," from which 

 five canals arise ; one of these (tc) passes directly upwards to 

 the aboral surface, where it branches and opens to the exterior 

 by usually two openings ; two others pass downwards parallel 

 with the broad surface of the stoniodseuin (me) and end 

 blindly ; while the other two (Fig. 67, re) pass directly out- 

 wards in the transverse axis of the body and end at the base 

 of the tentacles, a short distance before their termination 

 giving off two branches, one on each side. These branches 

 soon divide and give rise each to two canals which commu- 

 nicate peripherally with canals running longitudinally below 

 the rows of cilia-plate*. 



These plates, which constitute the locomotor organs, are 

 arranged one above the other and are composed of fused cilia 

 arising from ectodermal thickenings. There are eight merid- 

 ional rows of these ciliated plates, and from the upper end of 

 each row a delicate groove lined with ciliated cells extends 

 towards the aboral pole, each groove uniting with an adjacent 

 one as they approach the pole, so that their number is 

 reduced to four (Fig. 67, eg}. These pass in upon the floor of 

 a dome- shaped cavity enclosed by fused cilia which arch 

 together at the centre but do not quite meet. The cavity thus 

 enclosed is somewhat broader in the sagittal than in the 

 transverse axis and contains the aboral sense-organ. The floor 



