TYPE ANNELIDA. 



of Lumbricns, for instance, which possess a terminal funnel, and are like- 

 wise similar in structure to and may be regarded as repetitions of the 

 head-kidney, thus establishing the homology between the two forms of 

 nephridium. 



In carrying the homology of the Annelid nephridium back to that of 

 the Platyhelmiuth the question arises whether it is equivalent to the whole 

 branching system of the Turbellariau or only to a part of it. Whichever 

 view of metamerism be taken the various nephridia of the Annelids are to 

 be regarded as bud-products, and each, therefore, equivalent to a branching 

 Turbellarian nephridium. It has been suggested that the Annelid uephrid- 

 ial system has been produced by the fragmentation of an originally con- 

 tinuous system, but for this there is no embryological evidence. Each 

 uephridium being a bud from an undifferentiated nephridial blastema is 

 just as much an organ-individual as is the branched nephridium of a Tur- 

 bellarian just as much an individual, though of a lower grade, as is the 

 bud of a Polyzoon developed from an undifferentiated blastema. It 

 might be supposed, then, that the Annelid nephridium might show a 

 branched structure in certain primitive forms, and indeed a branched 

 head-kidney occurs in Polychset Trochophores. A branched condition is, 

 however, rare in the trunk nephridia, though it does occur in certain ter- 

 restrial Oligochaeta in which, however, it must be regarded as a purely 

 secondary phenomenon without any phylogenetic significance, since in the 

 development of such nephridia a single tube is first formed which later on 

 becomes solid and then gives off the branches, the various nephridial 

 branches of successive segments becoming sometimes united. This 

 branched condition passes into one in which the various branches separate 

 and acquire independent openings ; several pairs of nephridia, four iu the 

 anterior segments of a species of Periclmta and a greater number in other 

 forms, occurring in a single segment. This branching and multiplication 

 of nephridia is confined to terrestrial forms which in their conditions of 

 existence are farthest removed from the primitive state, and it is not 

 improbable that the multiplication bears some relation to the assumption 

 of a terrestrial mode of life. In the genus Lumbricus, in which the 

 nephridia are simple coiled tubes, a duplication of the nephridia in some 

 segments is to be found. The reproductive ducts are probably modified 

 nephridia, and in all aquatic forms other nephridia are absent in the 

 metameres in which they occur. In Lnmbricus, however, in, for instance, 

 the metamere which contains the oviducts, two pairs are present, one of 

 which retains its original excretory function, while the other has been 

 modified to form a duct for the reproductive elements. 



The reproductive organs have a very different arrangement 

 from what is found in the Polychaeta, being limited to a 

 comparatively few metameres ; and furthermore the Oli- 

 gochpeta are throughout hermaphroditic, the male and female 



