Echiuroids from Brackish Water. 
333 
generally. I give here a description of the system as it is to be seen in T. branchiorhyn- 
chus (fig. II) ; in the other two species it is essentially similar except for the relative 
lengths of the various vessels. A few variations in the connections of the vessels 
were also met with in some specimens; but this feature, as was noted by Jameson, 
is not a character of any great importance. 
In this description I will begin with an account of the so-called heart and then 
go on to a description of the other vessels and their relations. The heart (H) is only 
the partly swollen basal part of the dorsal vessel [D.v) and lies in front of the pre- 
intestinal constriction ; the strnctare of this part is in no way different from that of 
the rest of the vessel. From the heart the dorsal vessel continues forwards ; while 
it receives the two branches of the so-called neuro-intestinal vessel [N.v) which circle 
round the alimentary canal in the situation of the pre- intestinal constriction to open 
into the heart at its posterior end. The dorsal vessel continues forwards from the 
body into the proboscis as the median dorsal vessel of that structure ; at the tip of 
the proboscis it bifurcates and the two branches, bending downwards, continue on 
either side as the lateral vessels [L.v.) of the proboscis. These lateral vessels unite 
near the base of the proboscis to form the ventral vessel {V.v). This vessel runs over 
the ventral nerve-cord, ending blindly at its tip a little distance from the posterior 
end. Very near its anterior end the vessel gives off either a single branch which 
bifurcates further on or two branches. These circle round the oesophagus to form the 
so-called ' muscle- ring ' {M.r) of Spengel and then unite to divide once again into two 
branches, which as already described open into the heart after going round the ali- 
mentary canal in the region of the pre-intestinal constriction. This vessel with its 
forkings is known as the neuro-intestinal vessel. 
I have nothing to add to the previous descriptions of the histology of the blood- 
vessels. 
The course of blood in this system can not definitely be understood with the 
present, state of our knowledge of these forms. 
The coelomic fluid, which is probably of far greater importance in the physiology 
of the animal than the blood, fills up the whole of the body cavity and the spaces in 
the proboscis described above. It contains a large number of coelomic corpuscles of 
the same nature as have been described for other species. 
Segmental organs. — These organs have been designated by various names — 
Nephridia, anterior nephridia, brown tubes, genital pouches, segmental organs, etc. 
Of all these names segmental organ seems to be the most appropriate both from the 
point of view of structure and homology, and is adopted here. 
In the group of species under consideration there are two pairs of these struc- 
tures (5.o) lying on the ventral surface one behind the other, a pair on either side of 
the nerve- cord opening to the exterior behind the level of the hooks. 
Each segmental organ (fig. 12) consists of a vesicle (F.), which differs slightly in 
shape in the three species, but is rather narrow and elongated, tapering to a blunt 
apex in all. The opening to the exterior is at the base, while the internal opening or 
that of the funnel {F) lies on the upper surface a little above the point of attachment. 
