. Comparative anatomy and physiology. 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 ; whe
. Comparative anatomy and physiology. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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