ASTEEOIDEA 625 



The sexes are, with rare exceptions, separate. The testes and ovaries 

 look much alike and consist of five pairs of racemose glands at the base 

 of the arms, which open to the outside through minute pores in the inter- 

 radii. Adjoining the stone canal is the cylindrical axial organ enclosed 

 in the axial sinus, a portion of the body cavity. An extension of the 

 axial organ forms a ring around the aboral side of the body cavity, from 

 which a strand goes to each reproductive gland. The axial sinus follows 

 this extension and also finally encloses the glands themselves. The 

 larval starfish is either a bipinnaria or a brachiolaria. In some star- 

 fishes the female forms a brood sac by bending the arms together, in 

 which the eggs are sheltered. 



Distribution and Habits. Starfish are common animals through- 

 out the world, being found in all seas from tide lines to very great 

 depths. They move slowly about on the bottom by means of their 

 ambulacra! feet and also to some extent by the movements of their arms, 

 often in schools, devouring the mollusks and crustaceans they meet in 

 their path. They are important enemies of the oyster fisheries. A star- 

 fish kills its prey by wrapping its arms about it and everting its stomach 

 over it (Fig. 970), and slowly digesting its soft parts. Those which have 

 no suckers on the tube feet, however, can 

 not do this and are not carnivorous but 

 feed on minute organisms which they swal- 

 low. Starfish possess great regenerative 

 powers and can replace lost arms. 



History. Starfishes were well Known 

 to Aristotle and also to the medieval 

 writers on animals. The scientific knowl- 

 edge of them begins with Linck in 1733, Fig . 970 Starfish feeding (Cam- 

 whose work was a standard until the time of 



Cuvier. In 1816 appeared Tiedemann's comprehensive description of the 

 anatomy of the animals, and this and Miiller and Troschel 's classification 

 (1842) laid the foundation of our present knowledge of the group. The 

 bipinnaria larva was discovered by M. Sars in 1829. The modern classifica- 

 tion is based upon that of Perrier (1875), and of Sladen (1889). 



The class contains 3 orders and about 1,000 species. 



Key to the orders of Asteroidea: 



a v Starfishes with prominent contiguous marginal plates along the sides of 

 the arms and with 2 rows of ambulacral feet in the arm. 



1. PHANEROZONIA 



a a Starfishes usually with long arms and without prominent marginal 

 plates; aboral body wall spiny ; ambulacral feet with suckers. 



6j Pedicellariae very rare or absent and not pedunculate 2. SPINULOSA 



&, Pedicellariae present and pedunculate .>. FORCIPULATA 



