91 
internal (with respect to the middle line of the gill arch) 
sides of the filaments. These are the afferent filamentar 
vessels (4, A. and B.), and they are connected with the 
afferent branchial vessels which run in the fused bases of 
the filaments outside the tunnel referred to. If, on 
the other hand, the system is injected from the dorsal 
aorta, a second series of vessels which run down on the 
outer surfaces of the filaments becomes visible; these are 
the efferent filamentar vessels, and they are vonnected 
with the efferent branchial vessels which run in the tunnel 
on the convex surface of the arch. At regular intervals 
along its course the afferent filamentar vessel gives off an 
arterial twig on either side of the axis of the filament 
which passes into the respiratory lamelle (4, B). 
Text-fig. 4, B. represents a surface view of two 
lamelle, the transverse section of the filament passing 
through the axis between two such lamelle. In a 
fortunate injection of the branchial system it will be seen 
that the lamellar branches of the afferent filamentar vessel 
on entering the lamelle immediately break up into very 
close capillary networks. This capillary network, seen 
from the side, is represented by the transverse black lines 
connecting the two filamentar vessels in 4, A. Hach 
lamella has a wall which at the base is composed of cubical 
cells, but which over the flat surfaces is a thin squamous 
epithelium. Within the space enclosed by this wall is the 
capillary network, and no other tissues. According to 
Plehn* the blood flows in spaces hollowed out of adjacent 
closely-fitting cubical cells. The capillaries have not the 
ordinary epithelial wall characteristic of such vessels. 
After having traversed this network the blood is received 
* Zum feineren Bau der Fischkieme. (Vorl. Mitth.) Zool., Anz, No. 648, 
24 Bd., pp. 439-443, 1901. 
