262
THE BIOLOGY OF EUPHAUSIIDS
ones, but there is an indication of two main blood flows, one in a
dorsal and one in a ventral sinus. The dorsal sinus is restricted to the
anterior region round the blood gland and extends posteriorly to the
anterior end of the pericardium (Fig. 98). Blood'from this region is
carried to the posterior ventral region of the thorax. The ventral sinus
collects the blood from the eyes, antennae, antennules, mouthparts,
thoracic legs, etc., all of it being carried to the posterior end of the
thorax.
Thus the blood from all regions of the body reaches the gills which
are modified epipodites of the thoracic limbs. I n Meganyctiphanes
norvegica there are only seven fully developed pairs of thoracic limbs,
the pair on the eighth segment not being developed. Functioning gills
are present on segments 2 to 8, the first pair of thoracic legs having an
undeveloped simple epipodite, even although there are afferent and
efferent branchial channels present (Fig. 98) ; the same number of gills
are present in other euphausiids but the degree of development differs
anteroposteriorly and between species. The anterior gills are single
limbed but posterior ones may comprise two or three limbs of filaments.
Histologically, the filaments are composed of large cells with large
granular nuclei which usually possess a number of nucleoli. The cell
cytoplasm is also granular and striations were observed, in gills of
M . norvegica, traversing the cells, most often from the walls adjoining
the afferent to those nearest the efferent veins. A well-developed
internal muscular system is present, originating around the branchial
channels and extending into the lamellae so that each filament has a
few muscle fibres running up its centre to its distal end.
The blood, having reached the posterior ventral region of the thorax,
enters the afferent branchial channels which lie between the efferent
branchial channels (Fig. 98) and the body wall. There are various
muscles associated with these channels and they, along with the action
of the pericardial wall, drive the blood into the gills. The afferent
branchial channels lead into the afferent branchial veins which are in
the gills. These by means of sub-branches carry the blood to each gill
filament. A sub-branch of the afferent vein carries the blood terminally
on one side of each filament and a sub-branch of the efferent vein
returns it down the other side of the filament (Fig. 99). Connecting
these two veins are numerous small vessels passing from the afferent,
round the circumference of the almost tubular filament, to the efferent
vein. The efferent branchial veins lead the blood to the pericardium via
the efferent branchial channels (Fig. 98).
A schematic representation of the circulation of blood in an euphausiid
is shown in Fig. 100. When the heart contracts blood is ejected from
THE BIOLOGY OF EUPHAUSIIDS
ones, but there is an indication of two main blood flows, one in a
dorsal and one in a ventral sinus. The dorsal sinus is restricted to the
anterior region round the blood gland and extends posteriorly to the
anterior end of the pericardium (Fig. 98). Blood'from this region is
carried to the posterior ventral region of the thorax. The ventral sinus
collects the blood from the eyes, antennae, antennules, mouthparts,
thoracic legs, etc., all of it being carried to the posterior end of the
thorax.
Thus the blood from all regions of the body reaches the gills which
are modified epipodites of the thoracic limbs. I n Meganyctiphanes
norvegica there are only seven fully developed pairs of thoracic limbs,
the pair on the eighth segment not being developed. Functioning gills
are present on segments 2 to 8, the first pair of thoracic legs having an
undeveloped simple epipodite, even although there are afferent and
efferent branchial channels present (Fig. 98) ; the same number of gills
are present in other euphausiids but the degree of development differs
anteroposteriorly and between species. The anterior gills are single
limbed but posterior ones may comprise two or three limbs of filaments.
Histologically, the filaments are composed of large cells with large
granular nuclei which usually possess a number of nucleoli. The cell
cytoplasm is also granular and striations were observed, in gills of
M . norvegica, traversing the cells, most often from the walls adjoining
the afferent to those nearest the efferent veins. A well-developed
internal muscular system is present, originating around the branchial
channels and extending into the lamellae so that each filament has a
few muscle fibres running up its centre to its distal end.
The blood, having reached the posterior ventral region of the thorax,
enters the afferent branchial channels which lie between the efferent
branchial channels (Fig. 98) and the body wall. There are various
muscles associated with these channels and they, along with the action
of the pericardial wall, drive the blood into the gills. The afferent
branchial channels lead into the afferent branchial veins which are in
the gills. These by means of sub-branches carry the blood to each gill
filament. A sub-branch of the afferent vein carries the blood terminally
on one side of each filament and a sub-branch of the efferent vein
returns it down the other side of the filament (Fig. 99). Connecting
these two veins are numerous small vessels passing from the afferent,
round the circumference of the almost tubular filament, to the efferent
vein. The efferent branchial veins lead the blood to the pericardium via
the efferent branchial channels (Fig. 98).
A schematic representation of the circulation of blood in an euphausiid
is shown in Fig. 100. When the heart contracts blood is ejected from