Echiuroids from Brackish Water. 
335 
The vesicles open ventro-laterally. Their histological structure and probable 
function is treated at length further on. 
Lanchester in his description of the anal trees of T. sahinum says that they are 
short. In the specimen before me they measure 2 2 mm. in length, nearly one fifth 
of the total length of the animal. In the living spscimen they were tinged with yellow, 
while in the preserved condition they are of a creamy colour. The shape of these 
is shown in fig. 15. The funnels on the vesicles are not restricted to two lines as 
in the other two s.pecies, but their number is very much larger and they are arranged 
in a number of rows on the distal half or so. The funnels (fig. 15a) are much smaller 
than those of the other two species, being 0-076 mm. in length and the mouth 0-051 
mm. in greatest breadth. 
The wall of the vesicle is formed largely by flattened epithelial cells though a 
few muscle-fibres can also be distinguished here and there. The boundaries of the 
cells can not be distinguished in all cases. The protoplasm of the cells is granular 
and surrounds the rather small circular nucleus. The cells on the margins of the 
funnels and also on their walls are ciliated. The cilia are directed inwards and from 
the direction of their insertion it seems that they can work only inwards. 
Without going into details regarding the histology of the rectal portion of the 
intestine and the point where the anal vesicles open into it, it may be mentioned that 
the tissues are of the same nature as mentioned by Embleton (3) and Spengel (13). 
I could not find any cihated cells lining the openings of these vesicles into the rectum 
as are mentioned by Jameson for T. neptimi (6) and by Greef (4). The latter author 
has also described a special system of blood-vessels in the vesicle wall ; these vessels 
are according to Embleton not present in E. unicinctus and I have found no trace 
of them in my preparations ; and further there is no part of the blood-vascular 
system from which these blood-vessels could be given off as the dorsal blood-vessel 
does not extend so far back and the ventral one is solid in this region. Probably 
this special arrangement of the blood-vessels and the ciliated openings of these anal 
gills, as he terms them, led Greef to assign a respiratory function to these structures, 
which he considered to be analogous with the respiratory trees of the Holothurians. 
Schmarda ' and Forbes and Goodsir also held the same opinion. The structure of 
the vesicles, however, as I have interpreted it, does not permit this function to 
be assigned to these organs. The direction of the cilia in the funnels would not 
allow any fluid from the vesicles to enter into the bcrdy cavity. The contents of 
the vesicles are, moreover, coelomic corpuscles in various stages of degeneration, and 
not mud or sand such as would naturally be taken in with the incoming water 
from the rectum. The question of anal respiration in these forms has not received 
the attention it deserves and I am unfortunately not in a position at present to clear 
up the doubtful points. 
The structure and contents of the vesicles point on the other hand to an ex- 
cretory function, a function which has also been assigned to them by Huxley, 
1 Mem. Acad. Vienna, Vol. II, pis. iv-vii (1^52). 
