94 



SYSTEMATIC ZOOLOGY 



or bursae which extend into the body-cavity. These pockets are 

 connected with the reproductive organs, and frequently serve as 

 brood pouches in which the young undergo a part of their develop- 

 ment. The arms of the serpent stars are often transversely 

 banded in light and dark colors on their aboral surface, and this, 

 together with their writhing, serpentine movement, has suggested 

 the common name for these animals. The arms break very 

 readilv, thus furnishing the name, brittle stars. Instead of con- 

 taining a large body-cavity like the arms of the common starfishes, 

 thev are nearly solid, owing to the presence of the large calcareous 



FlG. 84. Ast common starfish on the northern 



portion of our Atlantic coast; specimen with regenerating arm, 

 01. 1 igraphed from a dried specimen by 



the author.) 



plates of the skeleton, which occupy the place of the ambulacral 

 grooves in the common stars. On this account the tube feet 

 cannot project from the oral side of the arms, but extend out 

 laterally and take no part in the locomotion of the animal, 

 being reduced to tentacle-like structures, without suckers at 

 their free ends, and without ampullae at their inner ends. The 

 arms generally bear spines which are situated along their lateral 

 borders. The reproductive organs and the digestive tract lie 

 exclusively in the central disc, and an anus is absent. 



The basket fishes differ from the brittle stars chiefly in haying 

 the arms branched dichotomously again and again, and these 



