CHAP. XV 



CTENOPHORA 



413 





(Fig. 180, M) a slight conical projection, and at the opposite 

 pole where the sense-organ is placed a slight depression (Ab). 

 In others, the sides of the body are drawn out into a pair of 

 wing -like lobes (Lobata), and the body is considerably flattened 

 cr compressed (Fig. 181). The Cestoidea have a long flattened 

 ribbon- or band-shape (Fig. 182), and the Platyctenea (Fig. 183) 

 are flattened in the oro- 

 apical axis and exhibit 

 a well-marked distinc- 

 tion between the dorsal 

 and ventral surfaces. 

 The shape of Beroe is 

 that of a hollow cone 

 or thimble. 



Ctenophoral plates. 

 In many Ctenophora 

 eight lines can be 

 traced, like the lines of 

 longitude on a globe, 

 from the area of the 

 sense-organ to the base 

 of the mouth-cone or 

 hypostome. In the 

 course of these lines 

 are situated the cteno- 

 phoral plates. In some 

 species they extend 

 along the greater part 

 of these lines of longitude, but in others they are more restricted. 

 That part of the line that bears the plates is called the " rib " or 

 " costa." These plates or combs form the principal organs of loco- 

 motion of the Ctenophores. They consist of a row of cilia fused 

 at the base (cf. p. 141) to form the plate, but free at the ex- 

 tremity where they form the comb-like edge. They are alternately 

 raised, by a rapid contractile action, and then slowly flattened 

 down again. The plates are raised in succession from the aboral 

 to the oral end of each rib, and the appearance given to the bands 

 in the living animal is that of a series of waves travelling down 

 the lines of longitude from the sensory area towards the mouth. 

 The effect of these rhythmic movements of the combs is to 



Fig. 180. Horniiphora plumosa. Ab, position of the 

 aboral sense-organ ; Ct, rib of ctenophoral plates ; 

 M, mouth ; t, tentacle, with two kinds of pinnae. 



(After Chun.) 



