2. THE SPECIES OF KRILL
7
varying between different genera ; the exopodites of these limbs produce
currents of water from which the food particles are filtered out by the
setae on the endopodites (Fig. 1). The first five abdominal segments
each have one pair of pleopods, the limbs with which the animal swims
while the sixth abdominal segment has no appendages (Fig. 1). The
two pairs of uropods along with the telson form a ( ( tail fan ”.
The first pair of abdominal limbs of the adult male are modified, in
all species except Bentheuphausia amblyops G. 0 . Sars to form a
copulatory organ, the petasma (Fig. l ) , whose form is usually diagnostic
of the species concerned. The copulatory organs of the female, the
thelycum, are also probably diagnostic of the species but not enough
data are available from all the species concerned to confirm this ; they
are located ventrally near the openings of the oviducts, which are medially placed on the sixth thoracic segment. Other features of diagnostic
interest are detailed in the legend to Fig. 1 and reference to this figure
should be made throughout the rest of this chapter.
The ‘( Introduction )’ began with an early reference to the food of
whales, ten years earlier, in fact, than the first scientific description of
an euphausiid by Milne-Edwards in 1830. This French zoologist
described a new genus of Crustacea having no gill cavities formed by the
sides of the carapace as in the decapod crustaceans-crabs, lobsters,
prawns, shrimps. He called the new genus Thysanopoda and its
representatives were characterized by the presence of sixteen gills
(eight pairs), each attached to the base of a thoracic limb and increasing
in size from front to rear, the ends of these gills floating freely in the sea.
He named the particular species he described as Thysanopoda tricuspide
(now known as T . tricuspidata) because it had three terminal spines on
its telson. Milne-Edwards noted the similarity of the bifid swimming
legs on the thorax of T . tricuspide to those of Mysis spp. and this is
undoubtedly the origin of the subsequent classification of the
euphausiids and mysids as the Schizopoda (divided feet), a classification
which will be discussed later.
Gudrin (1829-43) named Thysanopoda elongata and Krayer (1846)
named T . inermis, T . longicaudata, and T . neglecta, now all in the genus
Thysanozssa. Dana (1850) in his Synopsis Generum Crustaceorum
ordinis “Schizopoda )’ placed the Euphausidae in the subtribe Mysidacea
of the tribe Diploopoda belonging to the order Crustacea Schizopoda.
He described three genera of Euphausidae-Thysanopoda (M.-Edwards),
Euphausia (Dana), and Cyrtopia (Dana). The Cyrtopia turned out to be
larval stages of euphausiids and not a valid genus. He ascribed four
new species to the genus Euphausia; these were E . pellucida, E . splendens, E. gracilis, and the very important species E. superba. Two more
7
varying between different genera ; the exopodites of these limbs produce
currents of water from which the food particles are filtered out by the
setae on the endopodites (Fig. 1). The first five abdominal segments
each have one pair of pleopods, the limbs with which the animal swims
while the sixth abdominal segment has no appendages (Fig. 1). The
two pairs of uropods along with the telson form a ( ( tail fan ”.
The first pair of abdominal limbs of the adult male are modified, in
all species except Bentheuphausia amblyops G. 0 . Sars to form a
copulatory organ, the petasma (Fig. l ) , whose form is usually diagnostic
of the species concerned. The copulatory organs of the female, the
thelycum, are also probably diagnostic of the species but not enough
data are available from all the species concerned to confirm this ; they
are located ventrally near the openings of the oviducts, which are medially placed on the sixth thoracic segment. Other features of diagnostic
interest are detailed in the legend to Fig. 1 and reference to this figure
should be made throughout the rest of this chapter.
The ‘( Introduction )’ began with an early reference to the food of
whales, ten years earlier, in fact, than the first scientific description of
an euphausiid by Milne-Edwards in 1830. This French zoologist
described a new genus of Crustacea having no gill cavities formed by the
sides of the carapace as in the decapod crustaceans-crabs, lobsters,
prawns, shrimps. He called the new genus Thysanopoda and its
representatives were characterized by the presence of sixteen gills
(eight pairs), each attached to the base of a thoracic limb and increasing
in size from front to rear, the ends of these gills floating freely in the sea.
He named the particular species he described as Thysanopoda tricuspide
(now known as T . tricuspidata) because it had three terminal spines on
its telson. Milne-Edwards noted the similarity of the bifid swimming
legs on the thorax of T . tricuspide to those of Mysis spp. and this is
undoubtedly the origin of the subsequent classification of the
euphausiids and mysids as the Schizopoda (divided feet), a classification
which will be discussed later.
Gudrin (1829-43) named Thysanopoda elongata and Krayer (1846)
named T . inermis, T . longicaudata, and T . neglecta, now all in the genus
Thysanozssa. Dana (1850) in his Synopsis Generum Crustaceorum
ordinis “Schizopoda )’ placed the Euphausidae in the subtribe Mysidacea
of the tribe Diploopoda belonging to the order Crustacea Schizopoda.
He described three genera of Euphausidae-Thysanopoda (M.-Edwards),
Euphausia (Dana), and Cyrtopia (Dana). The Cyrtopia turned out to be
larval stages of euphausiids and not a valid genus. He ascribed four
new species to the genus Euphausia; these were E . pellucida, E . splendens, E. gracilis, and the very important species E. superba. Two more