23 NILS HJ. ODHNER, MOLLUSCA. 
emanate. The circulation within the gill of Chamidae, which is of the folded syn- 
aptorhabdic type, was then found to be as follows. 
The venous blood is carried out from two different sources into the gills, partly 
from the renal sinus and partly from the anterior blood lacunae in the visceral hump. 
From the renal sinus an afferent axial trunk passes into the branchial axis and runs 
down its whole length. It gives off laterals to the posterior gill only, and each 
lateral is enclosed in a principal filamentar septum ; further, we note the fact that 
each second septum contains an afferent vessel and each interjacent one an efferent. 
The blood from the afferent vessels is distributed through the interfilamentar junc- 
tions, and the filaments into those sides of a fold of the gill which incline towards a 
venous septum; the opposite fold sides again collect the blood that has been arter- 
ialized within the fold and carry it towards the efferent principal vessel. From 
here the blood is led by the arterial trunk into the gill axis, which debouches into 
the atrium of the heart. 
The afferent principal vessels of the posterior gill are distally joined to a marg- 
inal vein enclosed into the upper margin of the reflected gill lamella. This also 
emanates from the renal sinus. The efferent vessels, on the contrary, have no cor- 
responding distal connection, but taper and terminate towards the margin of the 
lamella. 
In the anterior demi-branch the conditions are somewhat different. All the 
afferent vessels here emanate from the upper margin of the reflected lamella, which 
encloses a vein, which is a continuation of the vena cava, which receives all the blood 
from the intravisceral lacunae and carries it backwards. The vena cava begins in 
the neighbourhood of the mouth, where an anterior vessel (from the adductor and 
the mantle), an inferior (from the foot), and a superior (from the umbones), join. 
— From the marginal vein of the anterior demi-branch, the afferent principal ves- 
sels plunge down, also here confined to each second principal filament. In this 
demi-branch the principal filaments of opposite lamellae join to interlamellar septa, 
which then contain the blood vessels, these being also here, as named, alternately 
afferent and efferent. It is a remarkable fact that the afferent septa stretch higher 
up towards the gill axis than do the interjacent efferent ones, so that thus the 
septa also alternate in length. 
The arterialization of the blood takes place into the same way as in the poster- 
ior demi-branch, in alternating synclinal folds, and the efferent septal vessels, which 
begin step by step under the free margin of the reflected lamellae and are not com- 
bined by any marginal vessel, debouch into the axial efferent trunk that carries the 
blood directly into the auricle. 
A comparison with the conditions prevailing in other Lamellibranchia would 
be beyond the limits of the present publication; only so much may be stated, that 
there are essential differences in the infra-branchial blood circuit as regards the dif- 
ferent types of branchiae. 
Also the extra-branchial or infra-visceral portion of the circulatory system in 
Chamidae presents some peculiarities not before known. From the anterior aorta 
