THE CTENOPHORA 



In the majority of the Ctenophora locomotion is effected solely 

 through the action of the combs of the costae. Only in the much 

 modified family of the Cestidae is the ciliary action supplemented 

 by sinuous movements of the elongated, band-like body. 



A costa is made up of a number of short transverse rows of 

 modified ectoderm cells, bearing exceedingly long cilia. The cilia 

 are fused together to form the swimming plate or comb. The 

 basis of each comb is a cushion composed of large columnar cells ; 

 these cells have broad bases and narrower ends, so that they con- 

 verge together (Fig. III. 5). According to Samassa, the ciliated 

 cells of one comb are in direct organic continuity with those of the 

 next succeeding comb by means of branched processes of the bases 

 of the cells, which processes traverse the intervening space, and admit 

 of stimuli being conducted from comb to comb (Fig. III. 4). The 

 cilia are borne on the narrower ends of the columnar cells, and are 

 fused to form a plate which is bent downwards at a tolerably sharp 

 angle at a short distance from the surface. When in action the 

 comb is straightened out so as to give a sharp stroke in an upward 

 that is, in the aboral direction, and then it swings slowly back 

 to the bent position of rest. The combs of each costa contract in 

 succession from the aboral towards the oral pole, their successive 

 action giving rise to the appearance of a wave travelling in the 

 same direction. It follows that the action of the combs drives the 

 animal through the water mouth forwards, its progress being just 

 the opposite to that of a Medusa. The activity of the combs of 

 each costa is directed and controlled by the aboral sense organ. 

 The structure of the latter shows it to be an organ of balance. If 

 the Ctenophore be tilted over to one side the otolith mass bears 

 down upon the balancer of that side, and the impulse thus originated 

 is transmitted from cell to cell of the ciliated furrows till it reaches 

 the first combs of the costae to which the furrows are distributed. 

 These combs immediately contract, and the stimulus is conveyed 

 from comb to comb by means of the processes of the ectoderm cells 

 described above. Thus the ciliated furrows function as nerves, 

 though they do not contain nerve fibres or nerve ganglion cells, 

 and the transmission of stimuli is effected by simple cell contact. 

 It must be borne in mind, however, that there is also a sub- 

 epithelial nerve plexus with ganglion cells and nerve fibrils, though 

 the latter are not known to be connected with the aboral sense 

 organ. 



The tentacles of the Ctenophora serve for the capture of prey, 

 and are not used in locomotion. They are most fully developed 

 in the Cydippidae (Hormiphora and Pleurobrachia) ; are present, 

 though much modified, in the Cestidae and Lobatae, but are absent 

 in the Beroidae. In Pleurobrachia and Hormiphora the tentacle, 

 consisting of a tentacular base and the tentacle proper, is retractile 



