LOWER INVERTEBRATES WHICH BREATHE IN WATER 415 



on either side by very numerous groups of pointed structures, 

 which are branches of the water-vascular system and act as gills. 

 This affords another instance of water-currents produced by ciliary 

 action having to do double duty, by bringing with them both food 

 and oxygen. Breathing is effected similarly in the fixed Sea- 

 Lilies, which are the most typical members of the group to which 

 the Feather-Stars belong. 



The delicate outgrowths just described are also of use as sense 

 organs. 



In ordinary Star-Fishes (Asteroids) the water-vascular system, 

 though retaining its uses in regard to breathing' and sensation, 

 has also acquired a new function, for it is here the means 

 of locomotion. The remote ancestors of these creatures were 

 almost certainly fixed forms (as Sea- Lilies still are), and when 

 these became free some means of moving about had to be evolved, 

 one solution to this problem being found in the way indicated. 

 On the under side of a Star- Fish five broad o-rooves are to be 

 seen, radiating from the mouth, and protruding from these are 

 numerous tube-feet, equivalent to the delicate projections which 

 flank the food-grooves in a Feather-Star. The walls of these 

 tube-feet are sufficiently thin to act as gills, though, as we have 

 seen, they are not the only, nor probably the chief, organs of 

 respiration in this case (see p. 413). 



Sea- Urchins (Echinoids). — In an 

 ordinary regular Sea- Urchin (see vol. i, 

 p. 456) of spheroidal shape there are 

 tube-feet comparable to those of a Star- 

 Fish, but in this case arranged along five 

 bands which stretch from one pole of 

 the sphere to the other. They can be 

 protruded beyond the tips of the spines 

 so as to enable the animal to walk or 

 climb, and they share the work of breath- 

 ine with the oral mils and the soft mem- 

 brane surrounding the mouth (see p. 413). 



Many of the Sea-Urchins, however, are of " irregular" shape, 

 being more or less flattened and markedly two-sided. One result 

 of this has been that the tube-feet present on the upper half of 

 the animal have become useless as locomotor organs. But they 

 have not been allowed to remain idle, for their value as breathing 



Fig- 544- — Upper Side of a Heart- 

 Urchin [Sfiata7igj(S p7irj>tircus^, show- 

 ing tube-feet specialized as gdls and 

 arranged like the petals of a flower. 



