4. THE LARVAE
129
E. krohnii, and E. recurva belong to Group A ; the first species has a
more oceanic distribution than the others and shows fewer variant
forms in its larval development. The larval development of eight
species in Group B is known and again the more oceanic species,
E. frigida, E. lucens, E. similis, E. vallentini, and E. tenera have the
fewest variant forms. Euphausia superba is an apparent exception but
this is because one large sample was different (Fig. 49) and, in general,
few variant forms are found in this species. Many of the slope species
extend into coastal waters and, referring to Fig. 49, it can be seen that
many more variant forms are present in the development of Meganyctiphanes norvegica in the St. Lawrence than in the more oceanic environment around Iceland. There is some doubt (see Bary, 1956) regarding
the true identification of the larvae named by Boden (1955) as
Euphausia lucens and so the development described by Boden for this
species has only been indicated (Fig. 47). Similarly, there is not enough
evidence to justify statements regarding the development of E. distinguenda and only one type of larva, that with one pair of setose and four
pairs of non-setose pleopods, has been recorded for E. hemigibba, the
second of the species investigated in Group C of this genus. The
remaining three species, oceanic in their distribution, belong to Group D
and their development is relatively constant (Fig. 47).
The genus Thysanozsssa comprises coastal, slope and oceanic species.
The two oceanic species, T . macrura and T . vicina, have a rigid pathway
of development. Thysanobsa longicaudata follows the same pathway of
development in oceanic areas (Einarsson, 1945) but a large number of
variant forms were present in the coastal area of the St. Lawrence
(Soulier, 1965a) although the dominant path of development was the
same as in the oceanic areas. Considerable variation in the development
of the other three species, especially in that of T . raschii (Fig. 49) has
been recorded. Thsre is, however, a strong tendency in the development
of species in this genus to have the same dominant pathway, namely, the
form with no pleopods moults to that with five pairs of non-setose
pleopods and so to the form with five pairs of setose pleopods.
Nematoscelis dificilis (Fig. 47) shows variation in its larval development whereas other species in this genus have more rigid patterns.
Sheard (1953) sampled the larvae of this species off Port Hacking and
once again the coastal environment of these larvae may be responsible
for the variation illustrated and a more rigid pattern of development
would be present in an oceanic area. Lebour (1926b,d) examined larvae,
which she identified as those of N . microps, near Alexandria, Egypt but,
according to Ruud (1936), this species does not occur in the Mediterranean. It is very difficult to distinguish N . microps from the closely
129
E. krohnii, and E. recurva belong to Group A ; the first species has a
more oceanic distribution than the others and shows fewer variant
forms in its larval development. The larval development of eight
species in Group B is known and again the more oceanic species,
E. frigida, E. lucens, E. similis, E. vallentini, and E. tenera have the
fewest variant forms. Euphausia superba is an apparent exception but
this is because one large sample was different (Fig. 49) and, in general,
few variant forms are found in this species. Many of the slope species
extend into coastal waters and, referring to Fig. 49, it can be seen that
many more variant forms are present in the development of Meganyctiphanes norvegica in the St. Lawrence than in the more oceanic environment around Iceland. There is some doubt (see Bary, 1956) regarding
the true identification of the larvae named by Boden (1955) as
Euphausia lucens and so the development described by Boden for this
species has only been indicated (Fig. 47). Similarly, there is not enough
evidence to justify statements regarding the development of E. distinguenda and only one type of larva, that with one pair of setose and four
pairs of non-setose pleopods, has been recorded for E. hemigibba, the
second of the species investigated in Group C of this genus. The
remaining three species, oceanic in their distribution, belong to Group D
and their development is relatively constant (Fig. 47).
The genus Thysanozsssa comprises coastal, slope and oceanic species.
The two oceanic species, T . macrura and T . vicina, have a rigid pathway
of development. Thysanobsa longicaudata follows the same pathway of
development in oceanic areas (Einarsson, 1945) but a large number of
variant forms were present in the coastal area of the St. Lawrence
(Soulier, 1965a) although the dominant path of development was the
same as in the oceanic areas. Considerable variation in the development
of the other three species, especially in that of T . raschii (Fig. 49) has
been recorded. Thsre is, however, a strong tendency in the development
of species in this genus to have the same dominant pathway, namely, the
form with no pleopods moults to that with five pairs of non-setose
pleopods and so to the form with five pairs of setose pleopods.
Nematoscelis dificilis (Fig. 47) shows variation in its larval development whereas other species in this genus have more rigid patterns.
Sheard (1953) sampled the larvae of this species off Port Hacking and
once again the coastal environment of these larvae may be responsible
for the variation illustrated and a more rigid pattern of development
would be present in an oceanic area. Lebour (1926b,d) examined larvae,
which she identified as those of N . microps, near Alexandria, Egypt but,
according to Ruud (1936), this species does not occur in the Mediterranean. It is very difficult to distinguish N . microps from the closely