486 J. S. HUXLEY AND G. R. DE BEER 



cytosis of the organism's own nematocysts would occur in its 

 normal life-history. It would thus appear that in certain 

 circumstances the power of ingesting food-particles, possessed 

 by certain types of cells, results in typical phagocytosis of 

 other cells of the organism — a process unusual or abnormal 

 for these forms, but usual and normal in higher animals. 



Loeb (1900) states that the tentacles fuse in some cases. 

 Thacher (1903) considers this to be only the appearance caused 

 by their being crowded together and by the ectoderm being 

 thrown into folds by excessive contraction. After careful 

 study of serial sections of a zooid in this condition we can state 

 that this explanation will not suffice. The tentacles are, it is 

 true, contracted and crowded, but there is actual fusion in 

 several places (PI. 26, fig. 9). 



The ectoderm cells of the tentacles and hypostome become 

 more cubical during resorption. 



Of the endoderm cells, the large mass of glandular cells in 

 the hypostome very soon disappears, the cells passing into the 

 cavity. 



In the last stage of resorption before the hydrotheca is 

 evacuated altogether (stage 5), what is left of the zooid is still 

 bounded by a definite epithelium (PI. 26, fig. 8) of flattened cells, 

 beneath which are others losing their differentiation. The 

 behaviour of the ectoderm cells may be compared with that of 

 a rear-guard, continually retreating yet always maintaining 

 an unbroken front. But as the zooid is resorbed, its volume 

 and surface decrease, and so the front diminishes. 



At the start the cavity is not too congested and the cells and 

 debris pass down ; but in later stages the cavity is almost 

 blocked up (PI. 26, fig. 8), and it is then that pulsation can be 

 observed. This obviously facilitates the evacuation. It is 

 presumably due to tension on the walls of the stolon. 



In Obelia it is possible to observe the cells actually leaving 

 the tissues, a process which has often been taken for granted 

 in other forms (see PI. 26, fig. 2, left side). 



Where do the resorbed elements go when they pass down 

 into the hydrocaulus out of the zooid ? It appears that they 



