322 Prof. M. M. Hartog on the True Nature of the 



term tlie " madreporic system." Claus, Gegenbaur, and 

 Huxley are all agreed on this point in their text-books. 

 Having had my attention early directed to similar statements 

 about the organ of Bojanus, and having been the first to demon- 

 strate * that this latter organ could not possibly take up water, 

 owing to the outward ciliary wash and the valvular orifice, I 

 was naturally inclined to doubt the received views on the 

 madreporite ; and latterly reflection on certain facts in vege- 

 table physiology induced me to inquire more fully into the 

 matter. 



The vegetable cell, containing in its cavities dissolved 

 substances of high osmotic equivalent, and bounded by proto- 

 plasm permeable to water but not to these substances, 

 tends to take up into its cavities an excess of water, limited 

 by various conditions which we need not discuss here ; and 

 thus the cell becomes turgescent, or erect as the animal physio- 

 logist would say. The animal body, with its system of 

 cavities and partially permeable walls, is in precisely the 

 same condition as the vegetable cell ; and if erection, turges- 

 cence, or dropsy do not occur when the body is immersed in 

 liquid (or air saturated with moisture), it is because of the 

 existence of a variously disposed apparatus through which 

 the excess of liquid is ejected, carrying off in solution various 

 soluble waste products. Such an apparatus is termed a 

 nephridium or kidney. 



We can see in Infusoria that when the contractile 

 vacuole fails to act with its habitual regularity under certain 

 abnormal conditions, the animal becomes dropsical, swells 

 up, and finally bursts. In higher animals we find either 

 ciliated funnels or special filter-pumps^ or both, acting to 

 remove the excess of liquid. If an erection in any part of 

 an animal be needed, the liquid can be supplied either 

 by the excess of endosmose over excretion, or by the flow of 

 liquid from one part to another. 



To these physiological considerations are added morpho- 

 logical ones of great significance. The accumulation of liquid 

 takes place into the coelom, in Annelids and Vertebrata a 

 mesothelial sac of which the first part of the nephridium is 

 a diverticulum^ to which an invaginated epihlastic duct is 

 added. The whole ambulacral canal-system of Echinoderm- 

 ata is a development of such a diverticulum of a mesothelial 

 cavity^ and the madreporic system is in great part at least an 

 epihlastic invagination. In the Echinopeedium we must 

 needs regard the madreporic system and the " vasal " part of 



* ' Journal of Anatomy and Physiology/ 1879. 



