46 THE AMPHINEURA 
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 trifureated. ach 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- 
Fie. 28. 
Ventral aspect of three species of Polyplacophora, showing the various sorts of gill-rows. 
A, Lepidoplewrus benthus ; B, Boreochiton cinereus; C, Schizochiton incisus. a, anus; f, foot; 
g, 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- 
branched 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 
