Echiuroids from Brackish Water. 
329 
a fairly thick layer of connective tissue cells and fibres, the latter more abundant, 
lying internal to the epithelium ; a few muscle fibres and nerves can also be seen 
in the connective tissue portion. Then there is the cavity of the gills which is in 
continuation with the spaces mentioned above. Further on the connective tissue 
layer is very much reduced and near the tips of the secondary branches a thin con- 
nective tissue lining is all that remains (fig. g). The cavity of these gills contains a 
large number of coelomic corpuscles, which shows that in the living animal the cavity 
contains coelomic fluid. The fluid in this portion of the gills would be separated 
from the surrounding water of the sea by the ciliated epithelium and the thin connec- 
tive tissue layer only. The structure of the gill-like processes of T. dendrorhynchus 
and T. sahinum is very like that of T. hranchiorhynchus, only they are less highly 
evolved. 
Besides the sinuses or spaces described above there are three blood-vessels in the 
proboscis. The median dorsal [D.v] lies deep in the connective tissue, the two 
lateral ones {L.v) in the outermost of the sinuses. Bach of these vessels has a distinct 
wall of its own, formed of an outer cubical epithelium, circular muscle-fibres and an 
internal endothelial lining. A few longitudinal muscle-fibres can also be distinguished 
lying internal to the circular layer, but they do not form a continuous sheath. The 
distribution and relations of the blood-vessels are described further on in the account 
of the blood- vascular system. It may, however, be mentioned that they form a 
definitely closed system, and there is no connection whatsoever between these blood- 
vessels and the coelomic spaces in the proboscis as was imagined by Greef (4), but 
denied by Spengel (13), and could not be seen by Reitsch (9). 
In transverse sections of the proboscis the lateral nerve [N .1) of each side is seen 
lying outside the lateral sinus. The distribution of the nerves is treated at length in 
the account of the nervous system. The minute structure of the nerves is the same 
as in the other Echiuroids. 
A few remarks regarding the function of the proboscis and the significance of the 
gill-like processes of these species may now be made. 
Greef assigned to the proboscis the function of a lung. The blood in its vessels 
being separated from the water of the sea by a thin tissue could, according to him, be 
easily oxygenated ; he also said that the proboscis was of use in the prehension of the 
food material, which it definitely siezed and rolled into the mouth. Of these func- 
tions the former one is not possible in the way Greef assumed, as there is no connec- 
tion between the blood-vascular system and the sinuses of the coelomic fluid, while 
the blood-vascular system is situated at a much deeper level. There is, however, as 
will be explained further, a great probability of the coelomic fluid being aerated here. 
The second function of the prehension of the food material is considered by 
Spengel to be performed only by the cilia on the ventral surface of the proboscis and 
not b}' the structure as a whole. Schmarda assigns to the proboscis of Bonellia 
viridis the function of respiration. Rolando ' remarks " es ist kein Anzeichen da, dass 
er ihm zum Athem oder als kieme diene." Embleton (3) is doubtful as to whether 
1 Isis von Oken j, pi. V, figs. 1-5 (1823). 
