2. THE SPECIES OF KRILL
11
critical consideration was given to the systematics of N . norvegica by
Holt and Tattersall (1905a) in their report on the schizopods of the
north-east Atlantic slope. They decided that the species differed
sufficiently from the others of the genus Nyctiphanes to warrant the
erection of a separate genus Meganyctiphanes to contain it. They
pointed out that, in Meganyctiphanes, the sixth and seventh thoracic
limbs of both sexes possess exopodites, whereas in female Nyctiphanes
they do not. Moreover, in Nyctiphanes, the peduncles, or stalks, of the
antennules are considerably stouter in the adult male than in the
female whereas the peduncles of male and female Meganyctiphanes are
similar. They further showed that the differences between the two
proposed genera regarding the possession or otherwise of exopodites
on the posterior thoracic limbs are probably associated with different
nursing habits ; in Nyctiphanes, the fertilized eggs are carried attached
to the underside of the female after she has laid them, whereas in
Meganyctiphanes they are liberated into the water, and exopodites
would be useless in animals which carry their eggs attached to these
posterior limbs because they would be unable to function. Hansen
(1 908a) accepted Holt and Tattersall’s new genus Meganyctiphanes
based on the differences between its females and those of the genus
Nyctiphanes and added a generic difference detectable in the males,
namely, the form of the clasping organs, known as the petasma, on the
first pair of pleopods. Those of Meganyctiphanes are essentially similar
to those of Thysanopoda and differ from those of Nyctiphanes.
It is not surprising to find in the early history of studies on any
species occurring over a fairly wide geographical area that individual
specimens were not always recognized as being identical, but as scientists become more familiar with the species this confusion is less likely
to occur. Meganyctiphanes norvegica is no exception and Michael Sars
himself described the species Thysanopoda nana (Sars, 1864b) which
was subsequently recognized by G. 0. Sars (1883a) as being identical
with young specimens of T . norvegica. Euphausia bnei was a new
species of euphausiid described by Holt and Tattersall (1 905a) from a
single specimen but in a subsequent paper Tattersall (1911) admitted
that Hansen (1910) was, in fact, correct in reassessing this animal as a
small specimen (body length 10 mm) of Meganyctiphanes norvegica from
which the dorsal leaflet of the basal joint of the antennule had become
detached. The difficulty of identifying such animals, especially
damaged ones, is emphasized when it is realized that an experienced
and careful zoologist was misled by this small difference not only to
describe the animal as a new species but also to include it in a different
genus. Riggio (1905) described some crustaceans from the Straits of
11
critical consideration was given to the systematics of N . norvegica by
Holt and Tattersall (1905a) in their report on the schizopods of the
north-east Atlantic slope. They decided that the species differed
sufficiently from the others of the genus Nyctiphanes to warrant the
erection of a separate genus Meganyctiphanes to contain it. They
pointed out that, in Meganyctiphanes, the sixth and seventh thoracic
limbs of both sexes possess exopodites, whereas in female Nyctiphanes
they do not. Moreover, in Nyctiphanes, the peduncles, or stalks, of the
antennules are considerably stouter in the adult male than in the
female whereas the peduncles of male and female Meganyctiphanes are
similar. They further showed that the differences between the two
proposed genera regarding the possession or otherwise of exopodites
on the posterior thoracic limbs are probably associated with different
nursing habits ; in Nyctiphanes, the fertilized eggs are carried attached
to the underside of the female after she has laid them, whereas in
Meganyctiphanes they are liberated into the water, and exopodites
would be useless in animals which carry their eggs attached to these
posterior limbs because they would be unable to function. Hansen
(1 908a) accepted Holt and Tattersall’s new genus Meganyctiphanes
based on the differences between its females and those of the genus
Nyctiphanes and added a generic difference detectable in the males,
namely, the form of the clasping organs, known as the petasma, on the
first pair of pleopods. Those of Meganyctiphanes are essentially similar
to those of Thysanopoda and differ from those of Nyctiphanes.
It is not surprising to find in the early history of studies on any
species occurring over a fairly wide geographical area that individual
specimens were not always recognized as being identical, but as scientists become more familiar with the species this confusion is less likely
to occur. Meganyctiphanes norvegica is no exception and Michael Sars
himself described the species Thysanopoda nana (Sars, 1864b) which
was subsequently recognized by G. 0. Sars (1883a) as being identical
with young specimens of T . norvegica. Euphausia bnei was a new
species of euphausiid described by Holt and Tattersall (1 905a) from a
single specimen but in a subsequent paper Tattersall (1911) admitted
that Hansen (1910) was, in fact, correct in reassessing this animal as a
small specimen (body length 10 mm) of Meganyctiphanes norvegica from
which the dorsal leaflet of the basal joint of the antennule had become
detached. The difficulty of identifying such animals, especially
damaged ones, is emphasized when it is realized that an experienced
and careful zoologist was misled by this small difference not only to
describe the animal as a new species but also to include it in a different
genus. Riggio (1905) described some crustaceans from the Straits of