chap, x.] MOVEMENTS OF ARTHROPODA. 375 



is spontaneously released " (Romanes and Ewart). 

 Ordinary Ophiuroids, which, according to the authors 

 just qiioted, are able to move along at the rate of six 

 feet a minute, have a certain wriggling power of 

 their arms, which, in the Astrophytidse, is converted 

 into a power of coiling for the purposes of attach- 

 ment, thanks to the fact that the faces of every one 

 of their arm joints are convex in one direction, and 

 concave in that which is at right angles to it. When 

 the spines are long, as in the piper (Dorocidaris), 

 where they are also of considerable stoutness, or in 

 Spatangus, where they are much more delicate, they 

 can be used as stilts, owing to the attachment of 

 muscular tissue to their bases. 



In the Artliropoda the function of locomotion, 

 like so many other functions in that group, falls very 

 largely upon the appendages, which may either act as 

 walking or as swimming organs. In the Crustacea, 

 where all but the first pair are typically biramose, 

 this locomotor function is seen in the early Nauplius 

 condition (see page 534), when even the antennae take 

 part in performing that duty ; these appendages, being 

 jointed and provided internally with muscles, are able 

 to move in various directions. At first, and in the 

 lower forms, they act more or less like oars, beating 

 the water as they move backwards and forwards. In 

 the higher forms, such, for example, as the crayfish, 

 the more anterior of the locomotor appendages act as 

 walking, and the more posterior as swimming organs. 

 In an appendage, which, has been but little 

 modified, and which may be regarded as typical, such 

 as the pair formed on the third abdominal segment, 

 we see a doubly-jointed basal piece or protopodite, 

 bearing two terminal pieces, the outer exopodite and 

 the inner endopodite. These pieces, which are fringed 

 with long bristles, or setae, are flattened, and can act 

 like oars. 



