37 4 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



sucker, and sway about or elongate their body so as 

 to reach their prey. In the flat-worms and in the 

 leeches there are longitudinal, circu]ar, and transverse 

 muscles. In all the rest of the Annulate, and 

 in the Gephyrea, there are only circular and longitu- 

 dinal bands in the body wall, the former of which are 

 the more external ; but, in addition to these, there are 

 smaller muscles which are of considerable importance 

 in locomotion, as they are inserted into the base of 

 the setse, and are the means by which these processes 

 are moved forwards and backwards, or used as parts of 

 a locomotor apparatus, working either as mere stilts as 

 in the earthworm ; or, as in the Polychseta, where they 

 are numerous, like oars in the free-swimming forms, 

 and as climbing hooks in those that live in tubes. 



Among the Ecliiiiodcrinata the most impor- 

 tant organs of movement are the contractile tube feet, 

 which are most valuable when, as often happens in 

 the Starfishes, or the Urchins, they are provided at 

 their free ends with a sucker-shaped enlargement by 

 means of which they can gain certain fixed 

 points to which they can draw their bodies. When 

 climbing up vertical, or almost vertical, heights, the 

 Echinoderm converts its pedicellarise (see page 297), 

 which are provided with special muscles, into organs 

 of locomotion, in so far that these pedicellarise seize 

 hold of waving fronds of sea-weed, which act, there- 

 fore, like the rungs of a ladder, up which one is 

 climbing by the use of the hands only ; it is of parti- 

 cular interest to observe that "the wonderfully tena- 

 cious grasp of the forceps is timed as to its duration 

 with an apparent reference to the requirements of the 

 pedicels (tube feet), for after lasting about two 

 minutes, which is about the time required for the 

 suckers (tube feet) to bend over and fix themselves 

 to the object held by the pedicellarise, if such should 

 be a suitable one, this wonderfully tenacious grasp 



