244 COMPARATIVE ANATOMY chap. 



Tridada and certain Nemertina) already shows a more or less distinct segmentation, 

 in that the efferent ducts are segmentally repeated. In all Plalodes and Nemertina, 

 however, longitudinal canals are present throughout the whole body or in the 

 anterior part of the body, which can open externally through more or less distinctly 

 paired and segmentally arranged ducts, whilst in all Annulata the nephridia are 

 separate at their first appearance, and with a very few exceptions (Laiiiee) remain 

 separate during life. While the water- vascular system of the Platodes is markedly 

 branched, such branching is less marked even in the Nemertina, and in the Annulata 

 {Oligochccta, Polychmta) is generally altogether wanting. This may be explained by 

 the fact that in the parenchymatous Platodes the excretory organs have to seek 

 out the excretory products all over the body, while the development of a blood- 

 vascular system and a body cavity affords spaces for collecting these products, out 

 of which the nephridia can take them direct. The nephridial funnel in the Annulata, 

 which ontogcnetioally originates quite separately from the other parts of the nephri- 

 dium, would then be a new adaptation, a collecting apparatus, suited for taking up 

 the excretory material out of the blood sinuses or body cavity, and for discharging 

 it through the nephridial canals. According to a second view, only the embryonic 

 head kidneys of the Annulata correspond with the water-vascular system of the 

 Platodes, Avith which they certainly often show a great structural resemblance. A 

 third supposition is that the head kidneys of the Chcetopoda and the embryonic 

 kidneys of the Hirudinea answer to the excretory organs of the Nemertina, while 

 the permanent nephridia may have arisen from the efferent ducts of the ovaries and 

 testes of the Nemertina. This last conjecture is opposed by the fact, which is 

 unanimously supported by all recent investigations, that the original function of 

 the Annulatan nephridium is excretory, and that only secondarily some of the 

 nephridia undertake the transmission of the sexual products. 



Prosopygia. — The number of the nephridia is everywhere in this 

 class very small ; there are never more than two pairs. In the Sipun- 

 culidce (Fig. 138, p. 208) there are two large tubular nephridia like 

 the permanent nephridia of the Folychceta, especially of the Echiuridm. 

 They emerge laterally at the limit between the proboscis and the 

 trunk (near the anus). Less frequently (PhascoUon) only one nephri- 

 dium occurs. The nephridia, besides their excretory functions, serve as 

 ducts for the transmission of the genital products. In Sipunculus 

 anal tubes which enter the hind-gut have been observed, which are 

 perhaps homologous with the anal tubes of the Echiuridce and the 

 Friapulidce. The Priapndidce have only two richly-branched anal tubes 

 emerging near the anus. At the blind ends of the branches are found 

 terminal cells mth long flagella projecting into the canals, simi- 

 lar to those which are characteristic of the water-vascular system of 

 the Platodes and of some of the embryonic head nephridia and the 

 provisional trunk nephridia of the Annulata. The anal tubes of the 

 PriapulidcB are said to act as excretory organs in youth, and in later 

 stages as places of formation and ducts for the transmission of the 

 sexual products. Phoronis possesses one pair of nephridia which open 

 outwardly and anteriorly by two lateral apertures, and besides their 

 excretory function also undertake the transmission of the genital 

 products out of the body cavity. Among the Bryozoa nephridia 

 have till now been found only in the Endoproda. They are paired 



