16
THE BIOLOQY OF EUE’XAUSIIDS
front edge of the first abdominal segment, or somite, an area of roughly
lenticular outline, which is entirely visible when the abdomen is flexed
under the body, and is occupied by a well-chitinized plate (Fig. 3b). I n
front, this plate is overlapped by the carapace and is connected to it by
a soft articular membrane and, behind, it is firmly joined to the front
of the first abdominal somite. Calman considered that this plate might
be the detached tergum (dorsal portion) of the last thoracic segment in
both euphausiids and mysids.
There is an apparent similarity between the gills of species of
euphausiids and species of the mysid families Lophogastridae and
Eucopiidae in that they all possess a single series on the thoracic limbs,
in contrast to several series in the Decapoda. However, the gills in
euphausiids correspond to podobranchiae, being borne on the exopodites
of the thoracic limbs, whereas those in mysids correspond to arthrobranchiae since they are attached to the joint membranes at the bases
of the limbs. These two types of gills, which may both occur in the
Decapoda together with pleurobranchs on the sides of the thorax, differ
greatly from one another in form.
There is no evidence a t any stage in the life of euphausiids of a
brood-pouch for holding the young, such as occurs in animals of the
orders Mysidacea, Cumacea, Tanaidacea, Isopoda, and Amphipoda,
which Calman grouped together in the division Peracarida. All animals
in this division also have attached to their mandible an accessory blade,
the Zacinia mobilis, not seen in the euphausiids or decapods, except
possibly in some larval stages where it may appear as a group of spines
which later disappears.
As will be seen later, Crustacea develop through a series of larval
stages punctuated by moults. Such development is inevitable in animals
with a hardened outer covering that also serves as their only skeleton.
These larval stages each have characteristic features and Calman
pointed out that there were close parallels in the larval development of
species of the Euphausiacea and of the Penaeidae. This latter group
consists of two families of deep-sea prawns living in similar habitats to
those of many euphausiids and they are definitely decapod crustaceans.
Many species of Sergestes, a genus of the penaeid family Sergestidae,
also bear photophores and so in some ways resemble the adult
euphausiids as well. Gelderd (1907) examined the gross and minute
structure and physiology of the digestive systems of a number of mysids
and of Nyctiphanes couchii and found them to differ. He observed that
the digestive gland in the euphausiid was of the decapod type and not
similar to that of the mysids, thus providing further evidence of the
true affinities of the euphausiids. Dr Isabella Gordon (1955a) discussed,
THE BIOLOQY OF EUE’XAUSIIDS
front edge of the first abdominal segment, or somite, an area of roughly
lenticular outline, which is entirely visible when the abdomen is flexed
under the body, and is occupied by a well-chitinized plate (Fig. 3b). I n
front, this plate is overlapped by the carapace and is connected to it by
a soft articular membrane and, behind, it is firmly joined to the front
of the first abdominal somite. Calman considered that this plate might
be the detached tergum (dorsal portion) of the last thoracic segment in
both euphausiids and mysids.
There is an apparent similarity between the gills of species of
euphausiids and species of the mysid families Lophogastridae and
Eucopiidae in that they all possess a single series on the thoracic limbs,
in contrast to several series in the Decapoda. However, the gills in
euphausiids correspond to podobranchiae, being borne on the exopodites
of the thoracic limbs, whereas those in mysids correspond to arthrobranchiae since they are attached to the joint membranes at the bases
of the limbs. These two types of gills, which may both occur in the
Decapoda together with pleurobranchs on the sides of the thorax, differ
greatly from one another in form.
There is no evidence a t any stage in the life of euphausiids of a
brood-pouch for holding the young, such as occurs in animals of the
orders Mysidacea, Cumacea, Tanaidacea, Isopoda, and Amphipoda,
which Calman grouped together in the division Peracarida. All animals
in this division also have attached to their mandible an accessory blade,
the Zacinia mobilis, not seen in the euphausiids or decapods, except
possibly in some larval stages where it may appear as a group of spines
which later disappears.
As will be seen later, Crustacea develop through a series of larval
stages punctuated by moults. Such development is inevitable in animals
with a hardened outer covering that also serves as their only skeleton.
These larval stages each have characteristic features and Calman
pointed out that there were close parallels in the larval development of
species of the Euphausiacea and of the Penaeidae. This latter group
consists of two families of deep-sea prawns living in similar habitats to
those of many euphausiids and they are definitely decapod crustaceans.
Many species of Sergestes, a genus of the penaeid family Sergestidae,
also bear photophores and so in some ways resemble the adult
euphausiids as well. Gelderd (1907) examined the gross and minute
structure and physiology of the digestive systems of a number of mysids
and of Nyctiphanes couchii and found them to differ. He observed that
the digestive gland in the euphausiid was of the decapod type and not
similar to that of the mysids, thus providing further evidence of the
true affinities of the euphausiids. Dr Isabella Gordon (1955a) discussed,