ASTERIAS COELOM 



437 



opened. This is a spacious cavity which apparently surrounds 

 the alimentary canal and extends into the arms. It has, how- 

 ever, its own proper wall, which is called the " peritoneum," both 

 on the outer side, where it abuts on the skin, and on the inner 

 side, where it comes in contact with the wall of the alimentary 

 canal. The outer wall is called the " somatic peritoneum," and it 

 is possible to dissect off the rest of the body- wall and leave it 

 intact ; the inner wall, from its close association with the 

 alimentary canal, is termed the " splanchnic peritoneum." This 

 wall can only be distinguished in microscopic sections from that 

 of the alimentary canal, to which it is closely applied. 



The coelom is filled with a fluid, which is practically sea 

 water with a little albuminous matter in solution. Through the 

 thin walls of the papulae oxygen passes into this fluid, whence it 

 easily reaches the inner organs, since they are all in contact 

 with some part of the coelomic wall. Similarly C0 2 is absorbed 

 by the coelomic fluid from all parts of the body, and diffuses 

 through the papulae to the surrounding water. 



The Starfish possesses no definite kidney for getting rid of 

 nitrogenous waste. In most of the higher animals with a well- 

 developed coelom it has been proved that the kidney is simply a 

 specialised portion of the coelom, and in many cases some parts 

 of the coelomic wall still retain their excretory functions, which 

 apparently the whole originally possessed. In the Starfish and 

 in Echinodermata generally this primitive state of affairs is still 

 retained. From the cells forming the coelomic wall, cells are 

 budded off into the fluid, where they swim about. These cells 

 from their movements are called amoebocytes. If a substance 

 such as indigo-carmine, which when introduced into the tissues 

 of the higher animals is eliminated by the kidney, is injected 

 into the Starfish, it is found soon after to be vigorously absorbed 

 by the amoebocytes. These later accumulate in the dermal 

 branchiae, through the thin walls of which they make their way 1 

 to the outside, where they degenerate. 



The coelom is indented by five folds, which project inwards 

 from the interradii. These folds are called the " interradial 

 septa " ; they are stiffened by a calcareous deposit, which is not, 

 however, sufficiently dense to constitute a plate. In one of the 



1 Durham, "Wandering Cells in Echinodermata," Quart . J . Micr . Sci. xxxiii. 

 1891, pp. 81 et seq. 



