64 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Astropliytidae) the .arms become more or less 

 branched. Lastly we have the class of the IIolo- 

 thuroidea, which are more nearly allied to the 

 Echinoids than to the Asteroids ; in these all signs of 

 the calycinal system have disappeared, the calcareous 

 skeleton is greatly reduced, and often consists merely 

 of scattered and minute calcareous plates, which are 

 sometimes altogether absent. In many cases the 

 tube-feet cease to be arranged in five regular rows, 

 and may, as for example in Synapta, disappear alto- 

 gether ; when this happens there remains no external 

 character which speaks to the five-rayed ancestry of 

 these extreme forms ; in other words, here again 

 external bilateral symmetry is re-acquired. Holo- 

 thuria, Cucumaria, Synapta, are the best known 

 examples of this group. 



It is impossible to escape from the belief that the 

 Arthropoda are more nearly allied to the Annulata 

 than to any other group of the worms, but they are 



Fig. 25. Peripatns capcnsis. 



Showing the elongated bilaterally symmetrical body, with the ringed antennse. 

 and the incompletely jointed paired appendages with a pair of terminal 

 claws. 



sharply distinguished from them by the fact that, in 

 all cases, one or more of the appendages of the body 

 are converted into organs which may be called mouth- 

 organs, jaws, or giiathites. Some idea of the primi- 

 tive form may be gathered from Peripatus, which 

 is the simplest Arthropod known to us. The body 

 was elongated, distinctly bilaterally symmetrical, the 

 prsestomium was provided with tactile antennae, and 



