4 6 



THE AMPH1NEURA 



behind the renal opening : it is the first to be formed and is the 

 starting-point from which the rest of the gills are added either 

 forwards or both forwards and backwards. Occasionally individual 

 gills may be bifurcated or trifurcated. Each gill has the typical 

 ctenidial structure, consisting of an axis bearing an anterior and a 

 posterior row of gill -lamellae or filaments. The blood from the 

 above-mentioned longitudinal vessel is distributed to each gill by 

 an afferent vessel running along the internal or pedal margin of the 

 axis, and, after being oxygenated in the lamellae, is carried back by 

 an efferent vessel running along the external or pallial edge of the 

 axis to another longitudinal vessel which conducts it back to the 

 corresponding auricle. 



3. Excretory Organs. There are two symmetrical kidneys, whose 

 relations were first discovered by Sedgwick. Each of them con- 



Fio. 28. 



Ventral aspect of three species of Polyplacophora, showing the various sorts of gill-rows. 

 A, Lepidopleurus bcnthus; B, Boreochiton cinerens ; C, Schizochiton incisus. a, anus; /, foot; 

 0, gills ; m, mouth ; pa, mantle ; pa', anal lobe of the mantle ; p.s, pallial slit ; te, pallial 

 tentacles. 



sists of an elongated renal canal, situated on the lateral side of the 

 visceral mass, and once folded on itself, so that its two ends are 

 posterior. The internal or dorsal end opens into the pericardial 

 cavity, through a ciliated aperture or funnel. The . external or 

 ventral end opens to the exterior, between two of the gills at the 

 hinder part of the body. The renal canal is dilated immediately 

 behind its external opening. It is excretory throughout its length, 

 and the excretory surface is increased by numerous small much- 

 br,anched caeca which lie close to the body -wall laterally and 

 ventrally and open into the canal (Fig. 29). 



Various kinds of kidneys are to be found (Plate). They 

 generally extend more or less forwards, and their extension is 



