282 WILLIAM BLAXLAND BENHAM. 
tenth of an inch throughout the greater part of the body, but 
which is modified anteriorly ; this is the dorsal trunk (D.). In 
many forms, e.g. Urocheta and Titanus, the dorsal vessel is, in 
some parts at least, ampullate, but in Microcheta this is not 
the case. This thin-walled tube may be traced forwards, till in 
somite x11r it becomes somewhat narrower, and continues 
with this less diameter till, in somite 1x, it enlarges again and 
becomes more muscular, whilst in somite vi1I is a very notice- 
able heart-shaped swelling (d. sw.). At first sight it appears 
to be merely an enlargement of the dorsal trunk with strong 
walls, but on opening it we find it contains a double lumen 
(Pl. XVI, figs. 15, 16). In fact the dorsal trunk has here 
divided into two more or less parallel vessels, lying quite 
close together, and having thick muscular walls; each of 
these vessels commences posteriorly with a narrow lumen, 
but as it passes forwards gradually enlarges and bulges ante- 
riorly, then very suddenly narrows again and unites with its 
fellow to form a single dorsal trunk, as in the posterior part 
of the somite. This single portion passes through the anterior 
septum into somite vi1, becoming much narrower, and then 
divides into two vessels parallel and close to one another, each 
of a little less diameter than the single portion from which it 
springs; these two again unite just behind the septum and pass 
through as a single trunk. The same thing happens in somites 
vi, v, 1v (dd., fig. 5), the dorsal trunk getting narrower and 
narrower. Thus we have, besides the enlarged double portion 
in somite vi1I, a narrow double portion of the dorsal trunk in 
somites VII, VI, V, Iv. This splitting of the dorsal trunk is 
described by Beddard in Pleurocheeta (36) and in Acanthodrilus 
sp. (40). In this paper he also describes the arrangement in 
Microcheta. In the somite 111 and onwards, anteriorly, the vessel 
lies on the pharynx, and it remains single, but divides just behind 
the cerebral ganglia into two vessels, one on each side, which 
pass downwards (d.r.), I am uncertain whether directly into the 
ventral trunk, or if they break up into a network in the 
pharynx, and thus are continued into the ventral trunk. 
Branches from the Dorsal Trunk.—In the region of the 
