18
THE BIOLOQY OF EUPHAUSIIDS
stages before metamorphosing to a protozoea which is an undoubted
sergestid larva.
All the above evidence supports a case for a close affinity between
the euphausiids and the ancestral stock of the Decapoda by way of the
penaeid family, Sergestidae. The earlier grouping of euphausiids and
mysids as the Schizopoda is now known to be artificial because their
characteristics are too diverse, but we must remember, as Waterman
and Chace (1960) have suggested, that euphausiids and decapods may
well have evolved from mysid-like types near the base of the malacostracan stock.
Euphausiids and decapods are now grouped together to form the
subclass Eucarida, characterized by the possession of a carapace which
is fused with all the thoracic somites (except possibly the hindermost
as Calman showed), stalked eyes, the absence of oostegites (ventral
flaps forming a ( ( brood pouch ”), a short heart in the thorax, and a
large branched digestive organ, the hepatopancreas. This subclass is
distinct from the Peracarida, with their brood pouches, posteriorly free
carapaces, more elongate hearts, and simple mid-gut caeca ; and from
the Syncarida, which lack a carapace or oostegites and possess simple
mid-gut caeca ; and from the Hoplocarida, which have a small carapace
fused with the first three thoracic somites, leaving four uncovered, the
first five pairs of thoracic limbs subchelate (with simple pincers) and
the last three pairs biramous, no oostegites, the first five pairs of
abdominal limbs bearing gills, and a large branched hepatopancreas ;
and from the Leptostraca which are characterized by a large carapace
not fused with any of the thoracic somites but attached to the sides of
the thorax by an adductor muscle, by seven abdominal somites, the
last having no appendages, and by the possession of tail branches on
the telson.
These subclasses together constitute the Malacostraca, all of the
more primitive members of which have in common those features known
as the earidoid facies. Generally speaking, malacostracan Crustacea are
characterized by the possession of compound eyes, usually borne on
stalks, a carapace over the thorax, a uniramous mandibular palp (not
always present), a thorax of eight and an abdomen of six (sometimes
seven) segments, all of which (except the seventh abdominal) bear
appendages, and a complex “ stomach ”, “ gizzard ”, or proventriculus.
We have now surveyed the general taxonomic relationships of the
euphausiids and progress to the next major question: how do we
distinguish the different species of euphausiids, one from another? The
initial problem is to ascribe the specimen to a genus and this can be
done by means of a key. The most important external characters used
THE BIOLOQY OF EUPHAUSIIDS
stages before metamorphosing to a protozoea which is an undoubted
sergestid larva.
All the above evidence supports a case for a close affinity between
the euphausiids and the ancestral stock of the Decapoda by way of the
penaeid family, Sergestidae. The earlier grouping of euphausiids and
mysids as the Schizopoda is now known to be artificial because their
characteristics are too diverse, but we must remember, as Waterman
and Chace (1960) have suggested, that euphausiids and decapods may
well have evolved from mysid-like types near the base of the malacostracan stock.
Euphausiids and decapods are now grouped together to form the
subclass Eucarida, characterized by the possession of a carapace which
is fused with all the thoracic somites (except possibly the hindermost
as Calman showed), stalked eyes, the absence of oostegites (ventral
flaps forming a ( ( brood pouch ”), a short heart in the thorax, and a
large branched digestive organ, the hepatopancreas. This subclass is
distinct from the Peracarida, with their brood pouches, posteriorly free
carapaces, more elongate hearts, and simple mid-gut caeca ; and from
the Syncarida, which lack a carapace or oostegites and possess simple
mid-gut caeca ; and from the Hoplocarida, which have a small carapace
fused with the first three thoracic somites, leaving four uncovered, the
first five pairs of thoracic limbs subchelate (with simple pincers) and
the last three pairs biramous, no oostegites, the first five pairs of
abdominal limbs bearing gills, and a large branched hepatopancreas ;
and from the Leptostraca which are characterized by a large carapace
not fused with any of the thoracic somites but attached to the sides of
the thorax by an adductor muscle, by seven abdominal somites, the
last having no appendages, and by the possession of tail branches on
the telson.
These subclasses together constitute the Malacostraca, all of the
more primitive members of which have in common those features known
as the earidoid facies. Generally speaking, malacostracan Crustacea are
characterized by the possession of compound eyes, usually borne on
stalks, a carapace over the thorax, a uniramous mandibular palp (not
always present), a thorax of eight and an abdomen of six (sometimes
seven) segments, all of which (except the seventh abdominal) bear
appendages, and a complex “ stomach ”, “ gizzard ”, or proventriculus.
We have now surveyed the general taxonomic relationships of the
euphausiids and progress to the next major question: how do we
distinguish the different species of euphausiids, one from another? The
initial problem is to ascribe the specimen to a genus and this can be
done by means of a key. The most important external characters used