4. THE LARVAE
117
calyptopes are similar in form, being setose and functional. Furthermore, the mouthparts of the calyptopes of all euphausiid species so far
examined are very similar indeed, although there is considerable
variation in the form of the adult mouthparts of different genera and
species (Mauchline, 1967b). The maxillipeds (or first pair of thoracic
limbs of the adult) are setose but are quite unlike the adult form.
The first calyptopis has an unsegmented abdomen (Fig. 44) although
often the abdomen is delineated from the cephalothorax. The carapace
is well developed and the free portion enlarged so that it hangs down
over the front and sides of the cephalothorax. The adult eyes are
beginning to differentiate but there is usually no trace of pigment or
cones and the median eye still persists, although it is absent in the
second calyptopis. Fraser (1936) found that the luminous organ of the
eye starts to develop in this stage in Euphausia superba as did Sars
(1885) who examined larvae of Nyctiphanes australis. Fraser also
describes two sensory organs in this region of the first calyptopis of
Euphausia superba but these have not been described in any other
species although they are probably present as are the two sensory setae
on the distal ends of the antennules, described by Sars in Nyctiphanes
australis and Fraser in Euphausia superba.
The second calyptopis (Fig. 44) differs from the first mainly in the
state of development of the abdomen which now has five segments, and
the uropods can be seen developing under the cuticle. The antennules
have developed flagella but the other appendages show few differences
from those of the previous stage. The eyes are better defined and the
first traces of pigment are often present. The second calyptopis of
Euphausia superba appears to be peculiar because, unlike those of other
species of euphausiids, buds, representing the early developmental
stages of the second to sixth thoracic limbs, are present in this larva ;
the thoracic limbs develop much earlier in this species than in others,
probably as an adaptation to the rigorous Antarctic environment in
which it lives.
The third calyptopis (Fig. 44) shows a further stage of development
of the abdomen which now has six segments and exceeds the carapace
in length. The carapace still covers the eyes which are becoming globular
in form, pigmented, and in which the visual elements can often be seen.
The uropods are developed but still very small while the thoracic legs
are still undeveloped but in E . superba there are, posterior to the first
pair of thoracic limbs, five pairs of bilobed buds which are the other
developing thoracic limbs (Fig. 44). The rest of the appendages of the
third calyptopis are similar to those of the second calyptopis and show
little advancement. The three calyptopes of any one species differ from
117
calyptopes are similar in form, being setose and functional. Furthermore, the mouthparts of the calyptopes of all euphausiid species so far
examined are very similar indeed, although there is considerable
variation in the form of the adult mouthparts of different genera and
species (Mauchline, 1967b). The maxillipeds (or first pair of thoracic
limbs of the adult) are setose but are quite unlike the adult form.
The first calyptopis has an unsegmented abdomen (Fig. 44) although
often the abdomen is delineated from the cephalothorax. The carapace
is well developed and the free portion enlarged so that it hangs down
over the front and sides of the cephalothorax. The adult eyes are
beginning to differentiate but there is usually no trace of pigment or
cones and the median eye still persists, although it is absent in the
second calyptopis. Fraser (1936) found that the luminous organ of the
eye starts to develop in this stage in Euphausia superba as did Sars
(1885) who examined larvae of Nyctiphanes australis. Fraser also
describes two sensory organs in this region of the first calyptopis of
Euphausia superba but these have not been described in any other
species although they are probably present as are the two sensory setae
on the distal ends of the antennules, described by Sars in Nyctiphanes
australis and Fraser in Euphausia superba.
The second calyptopis (Fig. 44) differs from the first mainly in the
state of development of the abdomen which now has five segments, and
the uropods can be seen developing under the cuticle. The antennules
have developed flagella but the other appendages show few differences
from those of the previous stage. The eyes are better defined and the
first traces of pigment are often present. The second calyptopis of
Euphausia superba appears to be peculiar because, unlike those of other
species of euphausiids, buds, representing the early developmental
stages of the second to sixth thoracic limbs, are present in this larva ;
the thoracic limbs develop much earlier in this species than in others,
probably as an adaptation to the rigorous Antarctic environment in
which it lives.
The third calyptopis (Fig. 44) shows a further stage of development
of the abdomen which now has six segments and exceeds the carapace
in length. The carapace still covers the eyes which are becoming globular
in form, pigmented, and in which the visual elements can often be seen.
The uropods are developed but still very small while the thoracic legs
are still undeveloped but in E . superba there are, posterior to the first
pair of thoracic limbs, five pairs of bilobed buds which are the other
developing thoracic limbs (Fig. 44). The rest of the appendages of the
third calyptopis are similar to those of the second calyptopis and show
little advancement. The three calyptopes of any one species differ from