SOME ASPECTS OF THE BIOLOGY OF THE CHAETOGNATHS
277
first is entirely on the trunk while the second pair may be partly on the
trunk section and partly on the caudal section. There are genera of
chaetognaths that have only one pair of lateral fins, as for instance
Eukrohnia, Krohnitta, Spaclella and Pterosagitta, while Sagitta always
has two pairs: these may sometimes be connected by a small band
made of the same tissue as the fin itself, as in the case of Sagittn lyra
Krohn.
Besides the lateral fins, chaetognaths also have a caudal fin. RGth
lateral and caudal fins are always horizontally disposed. The fins are
composed of two layers, one dorsal and one ventral, formed by a thin
epidermis beneath which there are at least two other layers of cells that
cover a furthcr epithelial plate which is the continuation of that which
is in contact with the musculature of the body,
An amorphous substance, which stains strongly with eosin, is
interposed between these two epithelial layers, dorsal and ventral, of
the fin. The thickness of this substance is not the same a t all points of
the fins: it is more abundant in the region where the fins are inserted in
the body, and becomes less towards the margin of the fin. The quantity
of this substance also varies with age: it is completely absent at the
moment of hatching, it increases with the progress of growth, and
decreases somewhat in the older specimens, in Spadella at least
(Ghirardelli, 1959a).
Both the lateral and caudal fins in Spadella are provided with very
thin rays 2 or 3 p thick : in the lateral fins the rays are parallel with one
another, 7 or 8 p apart, and sloping slightly backwards, while in the
caudal fin they spread fanwise (Ghirardelli, 1959a). John (1933) thinks
that these rays are formed from the amorphous substance and have a
supporting function, which had indeed already been suggested by
Grassi (1883). The amorphous substance itself probably also has a
supporting function, considering its elasticity, especially in Xagitta. In
Spadella a more valid support function can certainly be attributed in
the region of the fins to chordoid type cells (Grassi) which can be seen
to be similar to those described in connexion with the collarette. In
Spadella, furthermore, the rays of the lateral fins are not immediately
under the epidermis, as in Xagitta in which the epidermis is much
thinner and forms a layer of cells directly covering the amorphous
substance. Conditions similar to these can be found in Spadella only
in the widest part of the caudal fin, i.e. towards its free margin. In this
rcgion, the fin does not exceed 30 p in thickness and is formed by two
small epithelial plates, one dorsal and one ventral, covering the rays.
Under the rays, or rather interposed between the dorsal and ventral
rays, lies the amorphous substaiice about 8 p thick. An accurate
277
first is entirely on the trunk while the second pair may be partly on the
trunk section and partly on the caudal section. There are genera of
chaetognaths that have only one pair of lateral fins, as for instance
Eukrohnia, Krohnitta, Spaclella and Pterosagitta, while Sagitta always
has two pairs: these may sometimes be connected by a small band
made of the same tissue as the fin itself, as in the case of Sagittn lyra
Krohn.
Besides the lateral fins, chaetognaths also have a caudal fin. RGth
lateral and caudal fins are always horizontally disposed. The fins are
composed of two layers, one dorsal and one ventral, formed by a thin
epidermis beneath which there are at least two other layers of cells that
cover a furthcr epithelial plate which is the continuation of that which
is in contact with the musculature of the body,
An amorphous substance, which stains strongly with eosin, is
interposed between these two epithelial layers, dorsal and ventral, of
the fin. The thickness of this substance is not the same a t all points of
the fins: it is more abundant in the region where the fins are inserted in
the body, and becomes less towards the margin of the fin. The quantity
of this substance also varies with age: it is completely absent at the
moment of hatching, it increases with the progress of growth, and
decreases somewhat in the older specimens, in Spadella at least
(Ghirardelli, 1959a).
Both the lateral and caudal fins in Spadella are provided with very
thin rays 2 or 3 p thick : in the lateral fins the rays are parallel with one
another, 7 or 8 p apart, and sloping slightly backwards, while in the
caudal fin they spread fanwise (Ghirardelli, 1959a). John (1933) thinks
that these rays are formed from the amorphous substance and have a
supporting function, which had indeed already been suggested by
Grassi (1883). The amorphous substance itself probably also has a
supporting function, considering its elasticity, especially in Xagitta. In
Spadella a more valid support function can certainly be attributed in
the region of the fins to chordoid type cells (Grassi) which can be seen
to be similar to those described in connexion with the collarette. In
Spadella, furthermore, the rays of the lateral fins are not immediately
under the epidermis, as in Xagitta in which the epidermis is much
thinner and forms a layer of cells directly covering the amorphous
substance. Conditions similar to these can be found in Spadella only
in the widest part of the caudal fin, i.e. towards its free margin. In this
rcgion, the fin does not exceed 30 p in thickness and is formed by two
small epithelial plates, one dorsal and one ventral, covering the rays.
Under the rays, or rather interposed between the dorsal and ventral
rays, lies the amorphous substaiice about 8 p thick. An accurate