186
THE BIOLOGY O F EUPHAUSIIDS
gut which is short and into which open, ventrally, the two hepatopancreatic cavities and, dorsally, the two anteriorly projecting intestinal
caeca. The hepatopancreas has two lobes, a left and a right, and
produces the enzymes required to digest the food. The function of the
two intestinal caeca is unknown although they have glandular walls
and are thus likely to produce further digestive enzymes. The wall of
the mid-gut of Meganyctiphanes norvegica is composed of two layers,
the inner of high cylindrical epithelium, and the outer, bordering the
lumen, a clearly defined border of columnar cells. The epithelium is
bounded by a fine tunica propria and beyond this again is a layer of
circular muscle, a construction similar to that of the hind-gut wall
(Fig. 68). The mid-gut portion of the gut has no outer layer of longitudinal muscle and it extends posteriorly to about the region of the
centre of the heart. The high cylindrical epithelium is replaced by the
hind-gut epithelium but there is no point of obvious transition. The
hind-gut follows an almost straight course through the dorsal region
of the posterior thorax and abdomen to the caudal plate. Its epithelium,
however, is not of uniform structure but changes along its length (Fig.
68). The part of the hind-gut (Fig. 68) which extends from the junction
of the thorax and abdomen t o the posterior edge of the fourth abdominal
segment has a diameter which is greater than that of the previous part
(Fig. 68, c) and the epithelial cells become lower. The tunica propria is
no longer folded but is often pressed against and indistinguishable from
the band of circular muscle. The epithelium is of a high columnar type
(Fig. 68, e) and the diameter of the gut diminishes throughout the
anterior half of the fifth abdominal segment; the epithelium tends to
shrink away, in preserved animals, from the tunica propria thus leaving
the gap seen in the figure. The tunica propria is again folded and the
bands of circular muscle and connective tissue are well defined. The
next region, which extends through the posterior half of the fifth
abdominal segment, is characterized by an increase in the diameter of
the gut and a slight flattening of the epithelial cells (Fig. 68, g).
Vacuoles occur principally around the free edges of these ridges and are
probably the source of the material which binds the waste materials
together to form the faecal pellets. A faecal pellet examined under the
microscope has it fibrous appearance so that the waste material looks
as if it has been mixed up with cotton wool.
The functional aspects of these changes in the structure of the
hind-gut wall are not known with any certainty in euphausiids. The
food in the gut is moved posteriorly by peristalsis and in euphausiids,
the majority of which are transparent, these waves of motion are easily
seen. Retro-peristalsis (referred to by Vonk as anti-peristalsis) as well
THE BIOLOGY O F EUPHAUSIIDS
gut which is short and into which open, ventrally, the two hepatopancreatic cavities and, dorsally, the two anteriorly projecting intestinal
caeca. The hepatopancreas has two lobes, a left and a right, and
produces the enzymes required to digest the food. The function of the
two intestinal caeca is unknown although they have glandular walls
and are thus likely to produce further digestive enzymes. The wall of
the mid-gut of Meganyctiphanes norvegica is composed of two layers,
the inner of high cylindrical epithelium, and the outer, bordering the
lumen, a clearly defined border of columnar cells. The epithelium is
bounded by a fine tunica propria and beyond this again is a layer of
circular muscle, a construction similar to that of the hind-gut wall
(Fig. 68). The mid-gut portion of the gut has no outer layer of longitudinal muscle and it extends posteriorly to about the region of the
centre of the heart. The high cylindrical epithelium is replaced by the
hind-gut epithelium but there is no point of obvious transition. The
hind-gut follows an almost straight course through the dorsal region
of the posterior thorax and abdomen to the caudal plate. Its epithelium,
however, is not of uniform structure but changes along its length (Fig.
68). The part of the hind-gut (Fig. 68) which extends from the junction
of the thorax and abdomen t o the posterior edge of the fourth abdominal
segment has a diameter which is greater than that of the previous part
(Fig. 68, c) and the epithelial cells become lower. The tunica propria is
no longer folded but is often pressed against and indistinguishable from
the band of circular muscle. The epithelium is of a high columnar type
(Fig. 68, e) and the diameter of the gut diminishes throughout the
anterior half of the fifth abdominal segment; the epithelium tends to
shrink away, in preserved animals, from the tunica propria thus leaving
the gap seen in the figure. The tunica propria is again folded and the
bands of circular muscle and connective tissue are well defined. The
next region, which extends through the posterior half of the fifth
abdominal segment, is characterized by an increase in the diameter of
the gut and a slight flattening of the epithelial cells (Fig. 68, g).
Vacuoles occur principally around the free edges of these ridges and are
probably the source of the material which binds the waste materials
together to form the faecal pellets. A faecal pellet examined under the
microscope has it fibrous appearance so that the waste material looks
as if it has been mixed up with cotton wool.
The functional aspects of these changes in the structure of the
hind-gut wall are not known with any certainty in euphausiids. The
food in the gut is moved posteriorly by peristalsis and in euphausiids,
the majority of which are transparent, these waves of motion are easily
seen. Retro-peristalsis (referred to by Vonk as anti-peristalsis) as well