274
ELVEZIO GIIIXARDELLI
it differs greatly between one species and another. I n Sagitta friderici
Ritter-Zahony, for instance, the central region of the eye takes the
form of a square, and the pigment spot may be seen to allow the five
marginal clear cups. I n Sagitta hispida Conant the central region is
subrectangular, slightly curved and crescent-shaped ; the clear cups are
often difficult to see, four can be distinguished of which two are well
separated by the pigmented walls disposed crosswise, while the other
two are more or less attached to each other. Probably the eyes function
mainly as light receptors t o time the diurnal rhythms of the animal
and to govern any phototactic movement (Reeve, 1966) : actually the
major factor controlling the vertical distribution of Sagitta crassa
Tokioka in the day-time is the photosensitivity of the animal, which
nevertheless varies according to the growth stages (Murakami, 1959).
However, Singarajah (1966), in laboratory experiments, observes that
pressure increases cause vertical movements upward, but this response
to pressure is not affected by light and is abolished by small changes in
salinity, pH, temperature and violent agitation of the water.
Hooks or grasping spines (Hyman, 1959; Krumbach, 1903) and
teeth represent in some cases good systematic features: one or two
rows of teeth may enable the worker to differentiate among some
genera which are rather closely related, such as Eukrohnia which has
only the posterior teeth and Krohnitta which has only the anterior
teeth though well developed. I n every generation the numbers of
hooks and anterior and posterior teeth change as the body length
increases and also with the season (Murakami, 1959; Kado and
Hirota, 1957) ; the number of posterior teeth in S. crassa increases
suddenly when the food habit changes (Murakami, 1959). The sawlike hooks within the genus Xagitta are useful to distinguish the
different forms of Sagitta serratodentata Krohn from all other species
of the same genus, even if in this case, especially in adult animals,
other features also exist which are both surer and easier to observe.
Nevertheless Tokioka (1965a) does not regard the serrated feature
of the hook as of great systematic value, as, under high magnification, the hook edge in some species shows sometimes an insignificant array of faint prominences regularly or irregularly
arranged. Hooks and teeth are still very important characters in the
new classification of the chaetognaths proposed by Tokioka (1965a)
(see page 280.)
B. Integument, fins and tail
The integument may be composed of a rather smooth pluristratose
Furnestin (1967), using the electron microscope, confirms
epidermis.
ELVEZIO GIIIXARDELLI
it differs greatly between one species and another. I n Sagitta friderici
Ritter-Zahony, for instance, the central region of the eye takes the
form of a square, and the pigment spot may be seen to allow the five
marginal clear cups. I n Sagitta hispida Conant the central region is
subrectangular, slightly curved and crescent-shaped ; the clear cups are
often difficult to see, four can be distinguished of which two are well
separated by the pigmented walls disposed crosswise, while the other
two are more or less attached to each other. Probably the eyes function
mainly as light receptors t o time the diurnal rhythms of the animal
and to govern any phototactic movement (Reeve, 1966) : actually the
major factor controlling the vertical distribution of Sagitta crassa
Tokioka in the day-time is the photosensitivity of the animal, which
nevertheless varies according to the growth stages (Murakami, 1959).
However, Singarajah (1966), in laboratory experiments, observes that
pressure increases cause vertical movements upward, but this response
to pressure is not affected by light and is abolished by small changes in
salinity, pH, temperature and violent agitation of the water.
Hooks or grasping spines (Hyman, 1959; Krumbach, 1903) and
teeth represent in some cases good systematic features: one or two
rows of teeth may enable the worker to differentiate among some
genera which are rather closely related, such as Eukrohnia which has
only the posterior teeth and Krohnitta which has only the anterior
teeth though well developed. I n every generation the numbers of
hooks and anterior and posterior teeth change as the body length
increases and also with the season (Murakami, 1959; Kado and
Hirota, 1957) ; the number of posterior teeth in S. crassa increases
suddenly when the food habit changes (Murakami, 1959). The sawlike hooks within the genus Xagitta are useful to distinguish the
different forms of Sagitta serratodentata Krohn from all other species
of the same genus, even if in this case, especially in adult animals,
other features also exist which are both surer and easier to observe.
Nevertheless Tokioka (1965a) does not regard the serrated feature
of the hook as of great systematic value, as, under high magnification, the hook edge in some species shows sometimes an insignificant array of faint prominences regularly or irregularly
arranged. Hooks and teeth are still very important characters in the
new classification of the chaetognaths proposed by Tokioka (1965a)
(see page 280.)
B. Integument, fins and tail
The integument may be composed of a rather smooth pluristratose
Furnestin (1967), using the electron microscope, confirms
epidermis.