88 



permeable. Further work will be necessary to clear this question. 



2. The arguments collected by the Italian investigator R u s s o 

 110) to prove that the genital organs in Holothurians would 

 have an excretory function, have not convinced me. It may 

 be that one of the accessory functions (see p. 73) of these 

 organs is to store or possibly even to excrete certain substances, 

 but my solution seems to me to be the more probable one. 



In sea-urchins Giard 45) also observed crystals of calcium 

 phosphate in deutoplasmatic bodies in the genital organs. 



24. RESPIRATION, 

 a. General observations. 



It has repeatedly been suggested that the perivisceral fluid 

 might play the role of a carrier of oxygen, of an internal 

 respiratory medium. But this is only one of the numerous 

 guesses which have been made with regard to respiration in 

 Echinoderms, for no experimental work has ever been done 

 as far as the writer knows, to obtain more definite information 

 as to the reality of this function. 



But first let us see to what organs a respiratory function 

 has been attributed. In the starfishes we find the so, called 

 skin-gills (papulae), lacking only in one genus, Brisinga, which 

 are supposed to have respiratory importance. On their role 

 in excretion we spoke in the preceding chapter. They are 

 little bhnd sacs of the body wall, in which the coelomic fluid, 

 with its corpuscles, is kept in constant motion by the cilia of 

 the epithehal cells of the coelom. Peritoneum and epidermis 

 are here in contact according to K. C. Schneider; Delage 

 and Herouard state (p. 48) that occasionally muscle fibres 

 are found in the wall. Probably the ambulacral feet and 

 therewith the whole water-vascular system have a certain 

 importance in respiration, as later on we shall see. The con- 

 stant movements of these tube feet makes them very suitable 

 for such function, and in the chapter on excretion we have 

 mentioned other facts which make this still more probable. 

 Their walls are easily permeable. 



In many sea urchins the peristome is continued into 

 branched outgrowths, called internal gills or organs of S t e w a r t, 

 two in number, situated in each interradius. Every tube foot 

 is moreover connected by two canals with its ampulla, and 

 cilia cause a current up on one side, down on the other 

 side of the tube foot, thus allowing O2 to be taken up in 

 that way and CO2 to be given off by the periyisceral fluid. 

 I have already stated that some authors attribute respiratory 

 function to the accessory intestine of the urchins, but that this 

 did not seem to me very probable. 



In the brittle stars the so called bursa is supposed to be an 



