PHYSIOLOGY OF THE INVERTEBRATA. 219 



from tlie sea- water in which it lives, and to hold the oxygen 

 in its tissues. (4) In animal tissues chlorophyll or allied 

 pigments may be of use in furnishing oxygen to the animal ; 

 and in those Actinia- with "yellow cells" the chlorophyll 

 pigment appears to replace (more or less) the respiratory 

 proteid. (5) Throughout the whole animal kingdom, in each 

 tissue and organ there are present pigments which are con- 

 cerned in the respiration of those tissues and organs ; they 

 take the oxygen from the circulating blood, and fix it until 

 it is wanted for metabolic purposes in the cells. MacMunn* 

 calls all these coloured proteids histoha^matins, and the one 

 ftiund in muscle he named myoh?ematin. (6) Besides the 

 respiratory pigments found in the Actinia^ there are others 

 which appear to be of use for decorative purposes, and to this 

 class Moseley's actiniochrome belongs. Actiniochrome cannot 

 be changed into anything capable of being oxidised and 

 reduced ; in other words, it is not a respiratory pigment. 

 "What its use may be it is difficult to say. It may be intended 

 for a protective purpose or as a means of attracting prey. 



The Echixodermata. 



The cloacal tubes of the higher Holothurvha are most likely 

 of a respiratory function. These tubes, which ramify in the 

 perivisceral cavity, open by two orifices into the cloaca. This 

 cloaca receives the water, and projects it vigorously outwards, 

 on an average three times a minute. Analogous systems 

 exist in many other Echinoderms, and often they are bedecked 

 with cilia.t 



The ambulacral vesicles of other Echinoderms constitute 

 internal respiratory organs. Besides these organs, tissue- 

 respiration (by the aid of pigments) is well developed in the 

 Echinoderms. 



* "Researches on Myohasmatin and the Histohcematins," in Plillosophicnl 

 Transactions of Tioyal >'iocifti/, 18S6, pt. i. p. 267. 

 f Duges in his Physioloyie Coniparee, t. 2, p. 355. 



