60 University of California Publications in Zoology. [Vol. 4 



tion may be easily seen if one thrusts a pointed pipette con- 

 taining some finely ground India ink through the body wall of 

 the starfish and injects the ink into its body cavity. With a lens 

 the particles of the ink are seen circulating in the gills^. The 

 circulation is due to the lashing of fine cilia which line the in- 

 terior of the gills. 



On the outer surface of the gills the sea water likewise flows 

 rapidly toward the tip of the gills, driven by cilia on the surface. 

 Thus on the inside of the thin wall of the gill there is a continual 

 supply of new body fluid ; on the outside a continual fresh supply 

 of sea water, laden with oxygen. Through the thin membrane 

 the oxygen passes from the water into the body fluid, just as it 

 passes through the thin membrane of our lungs into our blood. 



This is evidently an effective method of obtaining fresh oxy- 

 gen, yet it exposes the starfish to various dangers. All its other 

 soft structures are protected by the hard spiny shell, but the 

 delicate gills are on the outside of the body. It is as if we had 

 our lungs outside instead of inside our bodies; evidently they 

 would be dangerously exposed. And in the starfish, as in our- 

 selves, the respiratory organs are most essential as well as most 

 delicate structures. They must be protected if the other life 

 processes are to go on. 



Serving mainly for the protection of these gills, the starfish 

 has developed a most extraordinary set of organs, known as the 

 pedicellariae. These, as is well known, are small jaw-like struc- 

 tures. If we examine closely the outer surface of the starfi.sh. 

 we find surrounding the base of each spine a fleshy ring, often 

 colored, in which are imbedded a large number of small whitish 

 oblong bodies having a cleft down the middle (fig. 1, p). These 

 are the pedicellariae. In the starfish at rest, these are shrunken 

 down into the flesh of the ring, and show little indications of 

 their real character. 



Now let us disturb a group of the gills of the starfish in some 



=* Ink or other foreign substances thus injected are later gotten rid of by 

 the gills in the following way. The ink particles, circulating in the gills, 

 are caught near the gill tips, so that in time the tips are filled by a black 

 mass. Now the tip of the gill becomes soft and gelatinous, and finally melts 

 away, allowing the ink to float off in long strings into the water. The gill 

 then closes at the tip. This remarkable process is easily observed; it has 

 been noted by others (Durham, 1888; Cuenot, 1902). 



