IV VERMES— NEPHBIDI A 241 



lie wholly in one segment, or else, as in tlie OUgochcda, each pair of nepliridia belongs 

 to two consecutive segments. 



Although there are many Polyelueta in which the pairs of nephridia are repeated 

 throughout the greater part of the body with great uniformity and in strictly seg- 

 mental arrangement, there are, on the other hand, many groups in which we find great 

 deviations from this arrangement. AVe can only mention the most important. 



In the CapitdlidcB permanent nephridia occur, as a rule, only in the abdominal 

 region, either in the greater part of that region or only in the anterior, or only in the 

 posterior portion. There is either one pair in each segment or several (Capitclla) ; 

 even as many as 6 pairs may occur in each segment. In most Ca/pitcllidce more or 

 less numerous pairs of nephridia are changed into genital tubes, which will be 

 further described below, and it is always the anterior pairs of nephridia which 

 undergo such a transformation. The permanent nephridia of Gapitella are distin- 

 guished by the fact that they possess, as a rule, more than one funnel. 



In the TerebcUoidea the nephridia only occur in the thoracic region, and in 

 strictly segmental arrangement. In this region the dissepiments are wanting, with 

 the exception of a strongly developed diaphragm which divides the ccelome of the 

 thoracic region into an anterior and a posterior cavity. The nephridia of the anterior 

 thoracic cavity function as organs of excretion ; those of the posterior conduct 

 the genital products to the exterior. In Lanicc conchiUga there is a very striking 

 nephridial arrangement. The 3 pairs of nephridia of the anterior thoracic 

 cavity do not emerge externally dii-eot, but the 3 nephridia of each side enter a short 

 nephridial duct which has a single external aperture. In a similar way the 4 

 nephridia of the posterior thoracic cavity enter on each side a longitudinal nephridial 

 duct, which, however, has not 1 opening but 4. It may therefore be said that 

 the 4 nephridia on each side are connected by a longitudinal canal. 



In the Cirratulidcc, Serpulacea, and Eermella (Fig. 147, p. 221) an anterior 

 sterile region and a posterior genital region may be distinguished, the genital products 

 attaining development in the latter. In the anterior sterile region, which consists 

 of a varying number of segments, only one pair of nephridia occurs. This pair alone 

 has an excretory function. They are long, and extend through several segments. 

 In the Cirratulidce they emerge ventrally by separate apertures in the third segment ; 

 in the Serpulacea and Hernwlla, however, they unite anteriorly to form an unpaired 

 duct, which reaches the exterior near the extreme anterior part of the body in the 

 dorsal middle line. In the genital region the nephridia are repeated in strictly 

 segmental arrangement, and serve as genital tubes for conducting the sexual products 

 to the exterior. 



In Sternaspis, two brown lobate bodies lying in the 5th and 6th segments are 

 regarded as nephridia ; these possess neither a lumen nor an internal aperture, and 

 end in the integument between the sixth and seventh segments. 



In the Echiuridce (Fig. 137, p. 207) there are two sorts of organs 

 which have been considered as nephridia, the so-called segmental organs 

 and the anal tubes. The segmental organs have quite the structure 

 of the permanent Polychsetan nephridia, and we can hardly doubt their 

 homology with the latter. They occur either in 2 pairs (EcJimrus), 

 or 3 pairs (Thalassema), or unpaired and singly (Bonellia); and 

 possess well-developed internal funnels. Their outer apertures lie 

 behind the anterior hooked setae. Their principal function is the 

 transmission of the genital products out of the body cavity. The 

 anal tubes are 2 long tubes which on the one hand enter the 

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