9. INTERNAL ANATOMY AND PHYSIOLOGY
265
into an almost unified mass (Chun, 1896). Six well-defined abdominal
ganglia are present, the commissures between the ganglia in Meganyctiphanes norvegica and Euphausia krohnii forming a single cord with no
gap present between them.
Euphausiids have a sinus gland and X-organ (Hanstrom, 1948).
The sinus gland, which is well developed in decapod crustaceans, is
present in euphausiids simply as a thickened disk on the neurilemma of
the medulla terminalis in the eyestalk. It lies next to a blood sinus but
is separated from it by a thin membrane ; its nuclei, which are relatively
few in number, resemble those of the neurilemma. The X-organ is also
in a simple form in euphausiids, obviously related to a sensory eye
papilla. It has groups of bipolar sense cells with oval nuclei a t its apex
and base and, between these groups, a group of secretory cells with
larger vesicular nuclei. Nothing is known about the hormones produced
by the X-organ-sinus gland complex of euphausiids nor of the role of
such hormones in activities such as control of chromatophore expansion,
retinal pigment migration, or moulting.
The musculature of Meganyctiphanes norvegica has been examined
by Daniel (1929) but that of other species of euphausiids has not been
studied. He states that his figures (Fig. 102) are diagrammatic
because he had difficulty in elucidating some of the junctions and some
of the bridges of muscle fibres linking certain muscles with each other.
The ventral musculature of the thorax is dominated by the large
thoracico-abdominal muscle (Fig. 102, No. I) which originates at the
anterior, lateral wall of the thorax, passes posteriorly and divides into
dorsal and ventral parts. The dorsal portion forms a saddle over
oblique muscle 1 (Fig. 102, No. 11) while the ventral portion enters the
first abdominal segment and, accompanied by the longitudinal part of
the oblique muscle I and several smaller muscles, reaches no further than
the posterior edge of the large transverse muscle of the segment. Oblique
muscle 1 (Fig. 102, No. 11) lies mainly in the thorax and has its origin
in the anterior end of the thoracic transverse muscle (Fig. 102, TT),
whence it travels anteriorly, then bends round and travels posteriorly
alongside the ventral part of the thoracic transverse muscle, terminating
with it opposite the anterior part of transverse muscle 1 (Fig. 102,
No. VI). The lateral thoracico-abdominal muscle lies dorsal to these
previous muscles in the thorax and is accompanied by the outer lateral
thoracico-abdominal muscle which originates at the membrane between
the thorax and abdomen and travels anteriorly throughout the greater
length of the thorax.
The abdominal musculature is very complex and Daniel’s description
should be consulted for further information. There are very powerful
265
into an almost unified mass (Chun, 1896). Six well-defined abdominal
ganglia are present, the commissures between the ganglia in Meganyctiphanes norvegica and Euphausia krohnii forming a single cord with no
gap present between them.
Euphausiids have a sinus gland and X-organ (Hanstrom, 1948).
The sinus gland, which is well developed in decapod crustaceans, is
present in euphausiids simply as a thickened disk on the neurilemma of
the medulla terminalis in the eyestalk. It lies next to a blood sinus but
is separated from it by a thin membrane ; its nuclei, which are relatively
few in number, resemble those of the neurilemma. The X-organ is also
in a simple form in euphausiids, obviously related to a sensory eye
papilla. It has groups of bipolar sense cells with oval nuclei a t its apex
and base and, between these groups, a group of secretory cells with
larger vesicular nuclei. Nothing is known about the hormones produced
by the X-organ-sinus gland complex of euphausiids nor of the role of
such hormones in activities such as control of chromatophore expansion,
retinal pigment migration, or moulting.
The musculature of Meganyctiphanes norvegica has been examined
by Daniel (1929) but that of other species of euphausiids has not been
studied. He states that his figures (Fig. 102) are diagrammatic
because he had difficulty in elucidating some of the junctions and some
of the bridges of muscle fibres linking certain muscles with each other.
The ventral musculature of the thorax is dominated by the large
thoracico-abdominal muscle (Fig. 102, No. I) which originates at the
anterior, lateral wall of the thorax, passes posteriorly and divides into
dorsal and ventral parts. The dorsal portion forms a saddle over
oblique muscle 1 (Fig. 102, No. 11) while the ventral portion enters the
first abdominal segment and, accompanied by the longitudinal part of
the oblique muscle I and several smaller muscles, reaches no further than
the posterior edge of the large transverse muscle of the segment. Oblique
muscle 1 (Fig. 102, No. 11) lies mainly in the thorax and has its origin
in the anterior end of the thoracic transverse muscle (Fig. 102, TT),
whence it travels anteriorly, then bends round and travels posteriorly
alongside the ventral part of the thoracic transverse muscle, terminating
with it opposite the anterior part of transverse muscle 1 (Fig. 102,
No. VI). The lateral thoracico-abdominal muscle lies dorsal to these
previous muscles in the thorax and is accompanied by the outer lateral
thoracico-abdominal muscle which originates at the membrane between
the thorax and abdomen and travels anteriorly throughout the greater
length of the thorax.
The abdominal musculature is very complex and Daniel’s description
should be consulted for further information. There are very powerful