246 



PHYLUM ECHINODERMA. 



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 coclom, 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. 



s.p. 



-P.C. 



t.m. 



FIG. 117. Diagrammatic cross-section of star-fish arm. 



After Ludwig. 



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

 septum in pseud-haemal vessel according to others ; 7c.r r . , radial 

 water vessel; am., ampulla; t.f., tube-foot; p.c., a pyloric 

 caecum cut across ; s.p., a calcareous spine : .<'" ^ skin-gill ; lac., 

 spaces in the wall ; gv., ova in ovary ; n.o., ambulacral 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 gonads develop periodi- 

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

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



