122
THE BIOLOGY O F EUPHAUSIIDS
either one or two moults depending on whether there is an instar
without pleopods.
PHASE 11. The furcilia with non-setose pleopods moults to a
larva with some pleopods setose and others non-setose. Usually
there is one moult but in some species the first instar in this phase
has one or two abdominal segments without pleopods and then two
moults are required to acquire five pairs of setose pleopods. Other
species, e.g. some Thysanoessa species, omit instars with combinations
of setose and non-setose pleopods, the instar with five pairs of nonsetose pleopods moulting to an instar with five pairs of setose pleopods.
PHASE 111. The furciliae all have five pairs of setose pleopods
and the terminal spines on the telson are not reduced in number.
The number of moults varies considerably-one to four.
The terminal spines on the telson are reduced in
number. There are usually three moults.
PHASE IV.
Anterior pairs of pleopods develop before posterior pairs of pleopods,
first appearing as non-setose pleopods (" buds ') or " rudimentary
pleopods ") which become setose and adult in form a t the next moult.
This is illustrated in Fig. 46. The third calyptopis moults to a larva
without any rudimentary pleopods or this larva can be omitted from
the developmental sequence and the calyptopis moults directly to a
larva with one or more pairs of non-setose pleopods. The next moult
produces a larva with the corresponding number of setose and nonsetose pleopods and all pleopods become setose a t the following moult.
Some species develop along a more or less constant pathway. For
example, many species in the genus Thysanopoda have calyptopes that
moult to furciliae with three non-setose pleopods, then to a larva with
three setose and two non-setose pleopods, and finally to a larva with five
pairs of setose pleopods and relatively few variations in this developmental pathway have been reported (Fig. 47). On the other hand,
species in the genera Nyctiphanes, Thysanoessa and Meganyctiphanes
can develop by several pathways (Fig. 47) and all combinations of
numbers of setose and non-setose pleopods may occur in larvae in the
same haul of plankton. This led to confusion in the early literature
because a larva with, for example, four non-setose pleopods was thought
to moult to one with five non-setose pleopods and so on in the following
manner
4' --f 5' + l"4' + 2"3' -+ 3"2' + 4"l' + 5".
Consequently, up to thirty moults were being postulated in the larval
development of any one species. Macdonald (1927b), working with
THE BIOLOGY O F EUPHAUSIIDS
either one or two moults depending on whether there is an instar
without pleopods.
PHASE 11. The furcilia with non-setose pleopods moults to a
larva with some pleopods setose and others non-setose. Usually
there is one moult but in some species the first instar in this phase
has one or two abdominal segments without pleopods and then two
moults are required to acquire five pairs of setose pleopods. Other
species, e.g. some Thysanoessa species, omit instars with combinations
of setose and non-setose pleopods, the instar with five pairs of nonsetose pleopods moulting to an instar with five pairs of setose pleopods.
PHASE 111. The furciliae all have five pairs of setose pleopods
and the terminal spines on the telson are not reduced in number.
The number of moults varies considerably-one to four.
The terminal spines on the telson are reduced in
number. There are usually three moults.
PHASE IV.
Anterior pairs of pleopods develop before posterior pairs of pleopods,
first appearing as non-setose pleopods (" buds ') or " rudimentary
pleopods ") which become setose and adult in form a t the next moult.
This is illustrated in Fig. 46. The third calyptopis moults to a larva
without any rudimentary pleopods or this larva can be omitted from
the developmental sequence and the calyptopis moults directly to a
larva with one or more pairs of non-setose pleopods. The next moult
produces a larva with the corresponding number of setose and nonsetose pleopods and all pleopods become setose a t the following moult.
Some species develop along a more or less constant pathway. For
example, many species in the genus Thysanopoda have calyptopes that
moult to furciliae with three non-setose pleopods, then to a larva with
three setose and two non-setose pleopods, and finally to a larva with five
pairs of setose pleopods and relatively few variations in this developmental pathway have been reported (Fig. 47). On the other hand,
species in the genera Nyctiphanes, Thysanoessa and Meganyctiphanes
can develop by several pathways (Fig. 47) and all combinations of
numbers of setose and non-setose pleopods may occur in larvae in the
same haul of plankton. This led to confusion in the early literature
because a larva with, for example, four non-setose pleopods was thought
to moult to one with five non-setose pleopods and so on in the following
manner
4' --f 5' + l"4' + 2"3' -+ 3"2' + 4"l' + 5".
Consequently, up to thirty moults were being postulated in the larval
development of any one species. Macdonald (1927b), working with