24 COMPARATIVE ANATOMY 



be deferred till the other organs of the body have been 

 considered. 



The excretory system of the earthworm is made up of a 

 number of pairs of coiled ciliated tubes opening by one end 

 into the coelom and by the other to the exterior. Such ciliated 

 tubes are known as nephridia, and as similar, though not 

 necessarily homologous, organs enter into the composition of 

 the excretory systems of most of the higher invertebrate and 

 vertebrate animals, it is important that their structure and 

 relations should be fully understood. In the common earth- 

 worm there is a pair of nephridia in every somite except the 

 first three and the last, and as all of them are similar in 

 structure it is not necessary to describe more than one member 

 of a pair. The nephridia may most conveniently be studied 

 in situ in the lyth and i8th somites, the gizzard and crop 

 being pulled over to the opposite side. The main part of a 

 nephridium consists of a tube coiled into several loops closely 

 bound together by membrane richly supplied with blood- 

 vessels and attached to the posterior face of a septum low 

 down and not far from the middle line. At the point of 

 attachment one end of the tube passes through the septum, 

 projects a little way into the cavity of the somite in front, and 

 opens into it by an expanded kidney-shaped funnel known as 

 the ciliated funnel or nephrostome. The other end of the 

 tube opens to the exterior by one of the minute nephridiopores 

 which have already been described as situated just in front 

 of the upper chseta of a ventro-lateral couple. Opening 

 internally in one somite, and externally in the next behind it, 

 a nephridium may conveniently be described as consisting of 

 a pre-septal and a post-septal portion. 



r ^ ne pre-septal portion includes the nephrostome and a 

 short length of a fine tube ciliated internally. The structure 

 of the nephrostome is rather complicated. Its body is formed 

 by a single large crescent-shaped cell known as the central 

 cell. The outer margins of the central cell are beset ""wtt'h" a 

 number of elongated wedge-shaped marginal cells, each with 

 a nucleus placed near its outer border. The marginal cells 

 are covered on one side with fine long cilia. The marginal 

 cells, diminished in size, pass round the horns of the crescents, 

 and running forwards become continuous with the edges of 



