2. THE SPECIES OF KRILL
17
at a meeting of the British Association, the importance of larval
characteristics in classification. She showed how the earlier ideas of
Calman had been developed by Kemp and by Gurney, taking as her
example, the Euphausiacea. She pointed out that both in the larval
development and in the structure of the adult animals there are indications of a close affinity between the euphausiids and the lower decapods,
especially the Penaeidae. The eggs of euphausiids and penaeids are
either shed freely or adhere for a brief period to the posterior thoracic
limbs whereas most decapods incubate their eggs which are attached to
the abdominal swimming legs. Euphausiids and penaeids are the only
Malacostraca with a free nauplius. The elongated, pear-shaped nauplii
of penaeids appear to be more primitive than those of euphausiids which
have a very abbreviated thorax, with the appearance of the second and
subsequent thoracic legs much delayed, except in the Antarctic species,
Euphausia superba, which acquires, precociously, the swimming and
feeding habits of the adult. Euphausiids, in having a gradual larval
development, resemble many penaeids, especially the Sergestidae, which
also undergo an abrupt metamorphosis at the end of each phase. I n the
sergestids, too, the last two pairs of thoracic limbs may be reduced or
absent, and in some species one pair of maxillipeds may be enlarged and
modified, as in euphausiids. Adult euphausiids and sergestids show
general similarities in the structure of the petasma and the spermatophores of the male, the thelycum of the female, and the photophores of
both sexes. I n Gordon’s view, resemblances between the animals of the
two groups far outweigh the differences ; the chief of these is that the
euphausiids have gills which are specialized podobranchs and are
exposed whereas in all the penaeids the gills are concealed by the
carapace, Lucifer being an exceptional penaeid genus in having no gills
at any stage of its life history. Gordon cited letters from both Calman
and Gurney on the relationships of the euphausiids. The former, in
1948, suggested making a group, the Euphausidea, co-ordinate with the
Penaeidea and the Caridea, presumably all together comprising the
Natantia, or swimming decapods, in contrast to the Reptantia or
creeping decapods, which include the crabs and lobsters. Gurney,
however, in 1949, favoured uniting the two groups which have a free
nauplius or protozoea, that is the Euphausiacea and Penaeidae, as one
suborder with the Decapoda as the other, Previously, in his Ray
Society monograph on decapod larvae, Gurney (1942) had suggested
dividing the Decapoda into two suborders, the Euphausiacea and the
Eudecapoda, but Gordon (1955b) considered his later suggestion to be
more acceptable. She further noted that a Japanese species of sergestid
has been shown to have a free nauplius which passes through three
17
at a meeting of the British Association, the importance of larval
characteristics in classification. She showed how the earlier ideas of
Calman had been developed by Kemp and by Gurney, taking as her
example, the Euphausiacea. She pointed out that both in the larval
development and in the structure of the adult animals there are indications of a close affinity between the euphausiids and the lower decapods,
especially the Penaeidae. The eggs of euphausiids and penaeids are
either shed freely or adhere for a brief period to the posterior thoracic
limbs whereas most decapods incubate their eggs which are attached to
the abdominal swimming legs. Euphausiids and penaeids are the only
Malacostraca with a free nauplius. The elongated, pear-shaped nauplii
of penaeids appear to be more primitive than those of euphausiids which
have a very abbreviated thorax, with the appearance of the second and
subsequent thoracic legs much delayed, except in the Antarctic species,
Euphausia superba, which acquires, precociously, the swimming and
feeding habits of the adult. Euphausiids, in having a gradual larval
development, resemble many penaeids, especially the Sergestidae, which
also undergo an abrupt metamorphosis at the end of each phase. I n the
sergestids, too, the last two pairs of thoracic limbs may be reduced or
absent, and in some species one pair of maxillipeds may be enlarged and
modified, as in euphausiids. Adult euphausiids and sergestids show
general similarities in the structure of the petasma and the spermatophores of the male, the thelycum of the female, and the photophores of
both sexes. I n Gordon’s view, resemblances between the animals of the
two groups far outweigh the differences ; the chief of these is that the
euphausiids have gills which are specialized podobranchs and are
exposed whereas in all the penaeids the gills are concealed by the
carapace, Lucifer being an exceptional penaeid genus in having no gills
at any stage of its life history. Gordon cited letters from both Calman
and Gurney on the relationships of the euphausiids. The former, in
1948, suggested making a group, the Euphausidea, co-ordinate with the
Penaeidea and the Caridea, presumably all together comprising the
Natantia, or swimming decapods, in contrast to the Reptantia or
creeping decapods, which include the crabs and lobsters. Gurney,
however, in 1949, favoured uniting the two groups which have a free
nauplius or protozoea, that is the Euphausiacea and Penaeidae, as one
suborder with the Decapoda as the other, Previously, in his Ray
Society monograph on decapod larvae, Gurney (1942) had suggested
dividing the Decapoda into two suborders, the Euphausiacea and the
Eudecapoda, but Gordon (1955b) considered his later suggestion to be
more acceptable. She further noted that a Japanese species of sergestid
has been shown to have a free nauplius which passes through three