CTENOPHORA 



413 



(Fio-. 180, J/) a slight conical projection, and at the opposite 

 ))ole where the sense-organ is placed a slight depression (Ah). 

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

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

 or 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 Bcroc is 

 that of a hollow cone 

 or thimljle. 



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- ^iq, iso.—Hormi2)hora plumosa. ^6, position of the 



phoral plates. In some aboral sense-organ ; Ct, rib of ctenophoral plates ; 



, , ,1 M, month ; t. tentacle, with two kimls of pinnae. 



species they extend (After Chun. l 

 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 comlvlike 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 



