112 GENERAL CONSIDERATIONS. 



instances in the animal kingdom of lowly organised solid 

 forms, which give a certain amount of justification to the 

 hypothesis of a solid anceitor. There is Trichoplax (No. 46), 

 the Orthonectidae, the whole of the Protozoa, the accelous 

 Turbellaria, and finally the Sponges, or at least some of 

 them under certain conditions. The latter case is of particular 

 interest, and deserves a little attention here. 



Metschnikoff (No. 38, p. 372) states that Halisarca, when 

 overfed with carmine, loses its canals and becomes a mass of 

 amoeboid cells containing swallowed food, and surrounded by 

 a common envelope of ectoderm. The same fact has been 

 observed by Lieberkiihn in Spongilla (No. 36), in winter. 

 From these observations, and others by HEeckel and Carter 

 (quoted by Metschnikoff, No. 38, p. 361), it appears that under 

 certain nutritive conditions, the flagellated endoderm cells of 

 sponges may lose their flagella and become amosboid, and the 

 whole sponge revert to the condition of the larva of Aplysiua 

 (Schulze, No. 47, PI. 24, fig. 30) of a protoplasmic network 

 with nuclei at the nodes, and a cortical layer of ectoderm. 



Metschnikoff (No. 38) has further observed that in many 

 cases the food of the sponge passes directly into the paren- 

 chyma and is not found in the collared endoderm cells ; while 

 in other cases it is found both in the cells of the endoderm 

 and parenchyma. These facts, as Metschnikoff points out, seem 

 to imply that the endoderm cells are not mainly nutritive, but 

 that their main function is to cause the currents through the 

 sponge body, and that the food brought by these currents 

 passes into the parenchyma, through the walls of the passages, 

 to be digested by the so-called mesoderm cells. 



The collared cells are thus inconstant, and appear to be 

 merely parenchyma cells specially modified under certain 

 conditions and capable of passing back into their original form 

 when the need for them has passed away. When they vanish 

 the canal system also goes and the sponge becomes solid so far 

 as the latter is concerned. Inasmuch as the parenchyma cells, 

 and probably also the cells of the ectoderm, are all connected 

 by their processes (except in the cases in which they break 



