310 



On the Nephridia of Perichaeta. 



[Jan. 12, 



be only one or two. The minute structure of the terminal section of 

 nephridia is slightly different from that of Acanthodrilus. I have 

 also found that Typhosus and a new genus, which I propose shortly 

 to describe under the name of Dichogaster, have many nephridiopores 

 in each segment. Another point, to which I wish to direct atten- 

 tion in this communication, is that in Perichaeta there is a connexion 

 between the nephridia of successive segments. 



Quite recently Ed. Meyer* and Cunninghamf have shown that in 

 Lanice concMlega the nephridia of each side are connected by a con- 

 tinuous longitudinal duct. This discovery is in accord with the pre- 

 sumed origin of the Annelid from the Platyhelminth excretory 

 system, and also with the development of Polygordius (Hatchek) and 

 Lwmbricus.% In Perichaeta the connexion between the nephridial 

 tufts of successive segments is not brought about by a continuous 

 longitudinal duct, one on each side of the body, but by numerous 

 tubules which perforate the intersegmental septum. Thus it appears 

 that the nephridial system of Perichaeta consists of a network of tubules. 

 In this respect Perichaeta agrees with the leech Pontobdella,^ but 

 differs in the presence of numerous nephridiopores in each segment. 



These facts appear to lend further support to the view that it is 

 possible to derive the annelid from the platyhelminth excretory system. 

 Lang has pointed out that the " secondary " pores by which the 

 excretory organ of the Platyhelminth s communicates with the exterior 

 have probably given rise to the nephridial pores in the Annelida; by 

 a subsequent arrangement of these in a metameric fashion, and by the 

 breaking up of the nephridial network, the paired nephridia have 

 originated. The longitudinal canal has disappeared, except in the 

 cases that I have already mentioned. In some Platyhelminths the longi- 

 tudinal canals are, partly at least, broken up into a network ; and it is 

 this condition which has persisted in Perichaeta and Pontobdella ; more- 

 over in some Platyhelminths, where the "secondary" pores have 

 become metamerically arranged, there are more than one pair to each 

 " segment." For this reason it is perhaps allowable to regard the 

 condition of the nephridia in Perichaeta as more archaic than 

 Pontobdella. The disappearance of the connexion between the 

 nephridia of successive segments leads to the condition which exists in 

 Acanthodrilus ; the reduction of the external pores, already perceptible 

 in the posterior segments of A. multiporus, culminates in the disap- 

 pearance of all but two in each segment. The irregularity in the 

 position of these, which is best marked in Plutellus, is the last trace 

 of the presence of multiple nephridiopores in each segment. 



# Quoted by Lang, " Die Polycladen." — Naples Monographs, 

 f ' Na'ure,' June 16, 1887. 



X Wilson, ' Journal of Morphology,' vol. 1, ~No. 1. 

 § 'Quart. Journ. Microsc. Sci.,' 1885. 



