Chap, x.] MOVEMENTS OF CCELENTERATA. 373 



In the Actiniae it is possible to distinguish a system 

 of longitudinal from one of transverse muscular fibres, 

 and the presence of these two explains how it is that a 

 sea-anemone is able, when irritated, to diminish both 

 in length and breadth ; the longitudinal muscles are 

 the best developed, and may be seen to be arranged in 

 definite bundles; the transverse are strongest in the 

 region of the base of the polyp (Fig. 54). The ten- 

 tacles owe their contractility to the possession of 

 muscular fibres. 



In the Ctenophora, which retain an external 

 investment of cilia along the lines of their " cteno- 

 phoral plates," the greater part in the production of 

 movements of the body is not effected by the muscles, 

 which are poorly developed in the ectodermal layer, 

 but by the contractile fibres which are developed in 

 the mesoderm, which is so richly developed in the 

 Ctenophora ; as seen in Beroe, these muscles are long 

 cylindrical cords, which are not united into bundles, 

 and are disposed radially, circularly, and longitudinally. 

 The greater number are, like Cydippe (Fig. 15), pro- 

 vided with a pair of long tentacles, in addition to which 

 other smaller or secondary tentacles may also be 

 present ; the axis of these is occupied by a cord of 

 muscular fibre; their most important office is, no 

 doubt, not one of locomotion, which is effected chiefly 

 by the ciliated paddles, but of prehension, for where 

 they are absent, the mouth is much wider than it is 

 in those that possess them. 



Many of the lower Worms move by the elonga- 

 tion of the anterior end of their body, which is suc- 

 ceeded by a contraction by means of which the hinder 

 part is brought to its original point of distance from 

 the anterior ; in the performance of this operation 

 they are sometimes aided by one or more cup-shaped 

 suckers, by means of which a fixed point is gained ; 

 others, like the leech, fix themselves by their hinder 



