80 COELENTERATA [CH. 



cells. In place of these there are gripping or adhesive cells 

 covered with a secretion by which they adhere to the prey. They 

 are often ripped off in the struggles of the prey. They are each 

 provided with an elastic tail embedded in the jelly which pulls in the 

 object to which they have adhered. The Ctenophora are free swimming 

 but their locomotion is performed not by the agency of muscular 

 bands but by eight bands of cilia which run like meridians of longitude 

 over the generally oval body from the mouth to the opposite pole 

 (5, Fig. 35). The cilia in each band are arranged in short transverse 

 rows, and the cilia in each row are joined at the base and free at 

 the tip. So each row has the form of a comb, and thus the name 

 Ctenophora, comb-bearers, is seen to be appropriate. Further the 

 principal sense organ is situated in the centre of the end of the 

 animal opposite to the mouth at the spot where the bands of 

 thickened ectoderm which carry the combs converge. These 

 thickened ridges of cells are often termed "ribs." If we compare 

 the animal to a globe, the end at which the mouth is may be 

 called the oral pole, the opposite end the aboral pole (2, Fig. 35). 

 The sense organ at the aboral pole is a plate of thickened ecto- 

 derm, the cells of which have developed nerve tails. Similar nerve 

 tails are developed by the bands of ectoderm which carry the combs. 

 Underneath the ectoderm in the spaces between the ribs there is 

 a network of fine nerve fibres derived from scattered nerve cells 

 like those of Hydra or Aurelia. The cells at the edge of the plate 

 carry cilia fused with one another which arch over the plate and 

 cover it like a tent. Inside is a calcareous ball supported on four 

 curved bars, each made of conjoined cilia, borne by some of the 

 inner cells. This ball acts as a balancing ' sense organ. If the 

 animal inclines to one side the ball will bear heavily on the support 

 on that side, and stimulate thus the corresponding ribs, which will 

 thus act more vigorously than the rest and tend to restore the 

 vertical position. 



Like Actinozoa, Ctenophora have a well-marked stomodaeum, and 

 the true coelenteron is represented by a series of branching canals, 

 the central one being termed the funnel (3, Fig. 35). The funnel 

 and stomodaeum are both flattened but in planes at right angles to 

 one another. The funnel gives off (1) two canals, the so-called 

 excretory canals, which open at the sides of the sense organ ; 

 (2) two canals, paragastric, running back towards the mouth 

 parallel with the stomodaeum (4, Fig. 35) ; (3) two canals running 

 each to the base of a branched tentacle, which can be retracted 



