xvii EXCRETION GROWTH OF POLYPIDE 473 



possible that the intertentacular organ (Fig. 234, t) described 

 on p. 508 may in some cases perform excretory functions. There 

 can, however, be little doubt that some kind of excretion takes 

 place in the Polyzoa ; and in considering what organs could 

 possibly perform this work, our attention is arrested by the 

 alimentary canal. The digestive organs of the young bud are 

 perfectly colourless. As growth proceeds, certain parts acquire 

 a yellowish, and later a brown colour. The degeneration of the 

 polypide is followed by the grouping of large numbers of the 

 free cells of the body-cavity into a mass which closely surrounds 

 the incipient " brown body." Under their action, the latter 

 becomes considerably smaller, probably as the result of the ab- 

 sorption of matters of nutritive value into other tissues. The 

 final result is the formation of the compact "brown body," 

 whose colour is principally derived from the pigment formerly 

 present in the alimentary canal. Experiments made by intro- 

 ducing into the tissues of the Polyzoa certain artificial pigments 

 which are known to be excreted by the kidneys when injected into 

 the bodies of other -animals, have given some reason for believing 

 that the appearance of the brown pigment in the wall of the diges- 

 tive organs is, in part, a normal process of excretion ; although 

 that process is not entirely carried out by the organs in question. 

 Little is known with regard to the duration of life of a single 

 polypide ; but some information bearing on this question may be 

 obtained from a set of observations made on Flustra papyrea. 1 

 The table gives the number of days from the time at which 

 the polypides were noticed to commence their degeneration : 



Days. 

 2 . . " Brown body" partially formed, the parts of the polypide being still 



easily recognisable. 



5 . . Tentacles still just recognisable : appearance of new polypide-bud. 

 8 . Stage shown in Fig. 235, 2. 



11 . Union of apex of stomach with " brown body." 

 16 . . "Brown body" half surrounded by stomach, and preparing to break 



up (Fig. 235, 3). 

 21 . . " Brown body " broken lip into numerous fragments, contained in the 



alimentary canal of the new polypide. 

 35 . . " Brown body " almost completely absorbed. 2 



1 Qiiart. J. Micr. Sci. xxxiii. 1892, p. 147. The experiment was conducted in 

 a laboratory, and the results may not be perfectly normal with regard to the time 

 occupied. 



2 See also Jolict, Arch. Zool. Exp. vi. 1877, p. 202, and explanation of plate 

 viii. for another series of observations. 



