BRANCHIURA SOWERBYI BEDDARD. 293 
The fibres are thus very conspicuous in sections through the posterior part of the 
body, and especially so is the largest fibre of the group (fig. 3), not only because of its 
actual size, but because, the whole section being so much smaller, it occupies relatively 
a far larger space than is the case anteriorly ; its diameter may be nearly as great as 
that of the whole nerve cord, and may actually be greater than that of the ventral . 
vessel close to it. 
I am inclined to think that these fibres, or at any rate the specially large one, were 
seen by Brepparp and described by him as the dorsal vessel. As will be seen, his 
description of the dorsal vessel is altogether at variance with the condition found by 
me, while it corresponds exactly to that of the large giant fibre. 
The dorsal vessel is described by Bepparp as being below the partition dividing 
the ccelom into upper and lower parts; as having thicker walls, and much less blood in 
the lumen, in sections, than the ventral; the blood in the dorsal vessel is, he states, 
never so darkly stained by carmine as the blood in the ventral vessel, to explain which 
he supposes that possibly the muscular walls of the dorsal vessel are particularly im- 
permeable to the staining fluid; when fully expanded the dorsal is stated to be of about 
the same calibre as the ventral vessel, but in certain parts its lumen was so contracted 
that the vessel could only with difficulty be recognised; the openings of the branchial 
vessels into the dorsal vessel were not seen, since at the point where these vessels 
should open it always happened that the dorsal vessel was very much contracted, while 
the end of the branchial vessel was much dilated (7c. the dorsal vessel, according to 
BEpparv’s interpretation, would thus have a regularly moniliform shape, being con- 
tracted almost to obliteration once in each segment). 
From my description, however, it appears that the dorsal vessel is above the incom- 
plete muscular partition of the ccelom, and has no close relation to the ventral nerve 
cord ; its lumen isin my specimens always full of blood, which has the same staining 
reactions as blood elsewhere (this, of course, is only what would be expected in sections 
stained on the slide); I found the dorsal to be often, if not usually, of greater calibre 
than the ventral vessel; the openings of the vessels into it were always well marked 
and patent, and there were no constrictions of importance along its course. 
The specially large giant fibre, on the other hand, is below the ccelomic partition, 
and has the relation to the nerve cord described by Bepparp for the dorsal vessel (on 
the left side just above the cord); the contents of the tube, which usually have the 
appearance of a coagulum, do in fact stain differently from the blood in the vessels; in 
diameter it is seldom (and only at the posterior end) as large as, or larger, than the 
ventral vessel; and at the septa it is regularly much constricted, so as to be, in the 
posterior portion of the animal, moniliform ; while, of course, no vessels are to be seen 
opening into it.* . 
* The appearance of giant fibres has led other observers also to consider them as tubes containing a coagulable sub- 
stance, e.g. SEMPER, who also found that the coagulum reacted to stains quite differently from blood-plasma ; in the 
crayfish the fibres have been held to be blood-vessels. Compare ASHWORTH (1), section u., “ Historical Account of the 
Giant Cells and Giant Fibres of Annelids.” 
TRANS. ROY. SOC, EDIN., VOL. XLVIII. PART II. (NO. 15). 45 
