ASTEROIDEA. 



229 



From the dorsal surface and sides of a star-fish in a pool, 

 numerous transparent processes may be seen hanging out 

 into the water. They are the simplest possible respiratory 

 structures, contractile outgrowths of the skin with cavities 

 continuous with the ccelom, and are called "skin-gills." 

 It is likely that pigmented cells of the body cavity fluid act 

 like rudimentary red blood corpuscles ; the water vascular 

 system may help in aeration ; and the whole body is, of 

 course, continually washed with water. 



i~M- 



Fig. 100. ■ 



-Diagrammatic cross section of star-fish arm.' 

 After Ludwig. 



Radial nerve ; b.v., radial blood vessel according to Ludwig, 

 septum in blood vessel according to others ; iv.v., radial water- 

 vessel ; a.m., ampulla ; t/., tube-foot ; p.c, a pyloric caecum cut 

 across; s.fi., a calcareous spine ; £-., a skin-gill ; lac., spaces in 

 the skin ; go., ova in ovary ; a.o., ambulacra! ossicle. 



The " skin-gills " are said to have an excretory function ; 

 for phagocytes, bearing waste, seem to traverse their walls. It 

 may also be that excretion is somehow concerned in forming 

 the carbonate of lime skeleton, but facts are wanting. 



The sexes are separate, and they are like one another, 

 both externally and internally. The organs develop period- 

 ically, and lie in pairs in each arm. Each is branched like 

 an elongated bunch of grapes, and is surrounded by a blood 

 sinus. Each has a separate duct, which opens on a porous 



