ARENICOLA. 229 
third diaphragm the gut swings freely until the beginning 
of the tail region, in which there are many septa. 
Vascular system.—The blood has a bright red colour, and is rich 
in hemoglobin. It flows in a very elaborate system of blood vessels, in 
regard to the details of which there is still some uncertainty. ‘here is 
along the whole mid-dorsal line of the gut a contractile dorsal vessel, 
which carries blood forwards from the seven posterior gills, etc. 
Connected with this by capillaries, there is below the gut an equally 
long, feebly contractile ventral vessel, which carries blood backwards 
to gills, nephridia, etc. Around the gastric region of the gut there is an 
elaborate plexus of blood vessels, which communicate by. two lateral 
vessels with the paired heart. There are also two sub-intestinal vessels 
between the ventral vessel and the gut; these lead through the plexus 
into the lateral gastric vessels, and thus into the hearts. These organs 
lie just behind the cesophageal glands, and consist on each side—(a) of 
a thin-walled auricle, an expansion of the lateral gastric vessel ; and (4) 
of a muscular ventricle, which drives the blood into the ventral vessel. 
Like the sub-intestinals, the dorsal vessel communicates with the heart 
only indirectly through the gastric plexus. The ventricle contains a 
spongy “‘ cardiac body,” which probably prevents regurgitation from the 
ventral vessel. 
From the ventral vessel arise afferent branchial vessels to gills, 
nephridia, etc. From the seven posterior gills efferent branches enter 
the dorsal vessel; while those from the six anterior gills join the sub- 
intestinals. Each efferent vessel gives off a branch to the skin, while 
the dorsal and sub-intestinal vessels give off numerous branches to the 
gastric plexus on the gut. 
Respiratory system.—There are thirteen pairs of gills, 
on the seventh to the nineteenth bristle-bearing segments. 
Each is a tuft of hollow thread-like branches, through the 
thin walls of which the red blood shines. The afferent 
branches to the gills all come from the ventral vessel; the 
first six efferent vessels from the gills open into the sub- 
intestinals ; the posterior seven open into the dorsal vessel. 
As the papillz on the proboscis are hollow and contain 
vessels, they are doubtless of respiratory significance. 
Indeed, the gills may be regarded as exaggerated papille. 
Excretory and reproductive systems.—In the anterior 
region, in segments 4-9, there are six pairs of nephridia, 
of which the foremost seems in process of degeneration. 
Each consists of three parts—a funnel opening into the 
body cavity, a glandular portion, and a bladder com- 
municating with the exterior. 
The sexes are separate and similar. The reproductive 
organs are very simple, and arise by proliferation of the 
