2i8
ELVEZIO GHIRARDELLI
description of the disposition of the fin rays was given by Grassi ; his
description refers to transverse sections of the lateral fins in which the
amorphous substance looks like a triangle with the shortest side in
contact with the musculature of the body.
Near the rays one sees elongated cells with very elongated nucleoli :
it is probable that they are cells of the inner (basal) epithelial plate that
have so modified. Everything points to the presumption that the rays
derive from these cells and that the amorphous substance also derives
from the epithelial plate.
Schmidt (1951), through observations with the polarizing microscope, has brought to notice in Xagitta setosu Muller and Xagitta
hexaptera d’0rbigny the presence along the rays of the fins of a substance
which shows the characteristic birefraction of lipoids.
The optical peculiarities of the fin rays lead further to exclusion of
the suggestion that they may be constituted of collagen. According t o
my researches and to the information supplied by the literature, a fin
of Spadella in transverse section, starting from the deepest layer, is
formed as follows :
(1) Two epithelial basal layers which are the continuation of the
epithelial layers in contact with the musculature of the body ;
between these two layers is the amorphous substance which is,
however, absent along the border of the fins and forms a middle
layer less extended than the fins themselves.
(2) Some layers of epithelial cells, more or less modified.
(3) Outside integument epithelium.
On the fins and specially on their dorsal surface there are glandular
formations and tactile prominences that have been accurately described
by Grassi (1883) and, as we have seen, accurately studied by Horridge
and Boulton (see p. 276).
The fins of chaetognaths are generally already present at the
moment of hatching but in the majority of the larvae of the genus
Xagitta it is not possible to make a clear distinction between anterior
and posterior fins.
The form and position of the fins constitute good systematic
characters, in well-preserved specimens, and so do the presence and
extension of rays on the fins. Even the extension of rays on the fins
may, however, show some variation between specimens of different
populations and also within the same population. Thus, for instance,
Xagitta minima is described as generally lacking rays on the anterior
fins, while in specimens in the north Adriatic the rays, though very
thin, are present on the posterior side of the anterior fin. Xagitta inJlata
ELVEZIO GHIRARDELLI
description of the disposition of the fin rays was given by Grassi ; his
description refers to transverse sections of the lateral fins in which the
amorphous substance looks like a triangle with the shortest side in
contact with the musculature of the body.
Near the rays one sees elongated cells with very elongated nucleoli :
it is probable that they are cells of the inner (basal) epithelial plate that
have so modified. Everything points to the presumption that the rays
derive from these cells and that the amorphous substance also derives
from the epithelial plate.
Schmidt (1951), through observations with the polarizing microscope, has brought to notice in Xagitta setosu Muller and Xagitta
hexaptera d’0rbigny the presence along the rays of the fins of a substance
which shows the characteristic birefraction of lipoids.
The optical peculiarities of the fin rays lead further to exclusion of
the suggestion that they may be constituted of collagen. According t o
my researches and to the information supplied by the literature, a fin
of Spadella in transverse section, starting from the deepest layer, is
formed as follows :
(1) Two epithelial basal layers which are the continuation of the
epithelial layers in contact with the musculature of the body ;
between these two layers is the amorphous substance which is,
however, absent along the border of the fins and forms a middle
layer less extended than the fins themselves.
(2) Some layers of epithelial cells, more or less modified.
(3) Outside integument epithelium.
On the fins and specially on their dorsal surface there are glandular
formations and tactile prominences that have been accurately described
by Grassi (1883) and, as we have seen, accurately studied by Horridge
and Boulton (see p. 276).
The fins of chaetognaths are generally already present at the
moment of hatching but in the majority of the larvae of the genus
Xagitta it is not possible to make a clear distinction between anterior
and posterior fins.
The form and position of the fins constitute good systematic
characters, in well-preserved specimens, and so do the presence and
extension of rays on the fins. Even the extension of rays on the fins
may, however, show some variation between specimens of different
populations and also within the same population. Thus, for instance,
Xagitta minima is described as generally lacking rays on the anterior
fins, while in specimens in the north Adriatic the rays, though very
thin, are present on the posterior side of the anterior fin. Xagitta inJlata