SOME ASPECTS O F THE BIOLOGY OF THE CHAETOGNATHS
273
A. The eyes, hooks and teeth
There are two small pigmented eyes on the head. These have a
central region composed of five combined pigmented cups (cupolae),
one of which is much larger than the others and occupies about one
half of the surface of the eye; each of the cups encloses a bundle of
visual cells in connexion with prolongations of the optical nerve,
connected with elements having a complicated structure.
The structure of the eyes has also lately been studied under the
electron microscope by Eakin and Westfall (1964). The electron
microscope reveals a star-like pigmented cell in the centre of the eyc
of Xagitta scrippsae Alvarico. Between the arms of the pigmented cell
there are masses of processes of photo-receiving cells. Each process
consists of a tubular segment containing some longitudinally aligned
microtubes each with a diameter of about 500 and a length of 20 p ;
a cone-shaped body composed of a core of large granules situated at thr
base of the tubular segment ; a connecting piece which has the appearance of cones and rods, and connects the processes of the sensory cells
with a fibrillar apparatus made up of nine peripheral double small tubes
(the central one is absent; the structure is of the type 9 + 0). Near
the connecting zone there is a small typical centriole with a striated
rootlet. The receptor process of the cell runs deep down into the sensory
cell which seems to possess a corona of microvilli on its inner surface.
A nervous fibril arises from the outer extremity of the cell and passes
into the optic nerve.
On the basis of these features, Eakin and Westfall also put forward
some suggestions about the phylogeny of chaetognaths which are
discussed below (see p. 363). The external aspect of the eye has been
used by some authors as a systematic character for the determination
of the species. Tokioka (1950) has compared the size and form of the
cye and of the inner pigment spot, and has established several proportions that might be useful for identification of poorly preserved specimens. Net0 (1961) also regards the eye characters as useful for species
determination. Similar work has been done by Fraser (1952) on
chaetognaths in Scottish waters ; although feeling some doubt as to the
usefulness of the method, he said that in some instances it can be used to
differentiate certain species. Also Furnestin (1954) has studied the eyes of
cleveii species of the eastern Atlantic. She has stressed that the generally
well-defined limits of the central region of the eye are constant in any
one species and may constitute a good classificatory character.
The same holds for the pigmented septa which separate the visual
cupolae : their arrangement varies very little within one species while
273
A. The eyes, hooks and teeth
There are two small pigmented eyes on the head. These have a
central region composed of five combined pigmented cups (cupolae),
one of which is much larger than the others and occupies about one
half of the surface of the eye; each of the cups encloses a bundle of
visual cells in connexion with prolongations of the optical nerve,
connected with elements having a complicated structure.
The structure of the eyes has also lately been studied under the
electron microscope by Eakin and Westfall (1964). The electron
microscope reveals a star-like pigmented cell in the centre of the eyc
of Xagitta scrippsae Alvarico. Between the arms of the pigmented cell
there are masses of processes of photo-receiving cells. Each process
consists of a tubular segment containing some longitudinally aligned
microtubes each with a diameter of about 500 and a length of 20 p ;
a cone-shaped body composed of a core of large granules situated at thr
base of the tubular segment ; a connecting piece which has the appearance of cones and rods, and connects the processes of the sensory cells
with a fibrillar apparatus made up of nine peripheral double small tubes
(the central one is absent; the structure is of the type 9 + 0). Near
the connecting zone there is a small typical centriole with a striated
rootlet. The receptor process of the cell runs deep down into the sensory
cell which seems to possess a corona of microvilli on its inner surface.
A nervous fibril arises from the outer extremity of the cell and passes
into the optic nerve.
On the basis of these features, Eakin and Westfall also put forward
some suggestions about the phylogeny of chaetognaths which are
discussed below (see p. 363). The external aspect of the eye has been
used by some authors as a systematic character for the determination
of the species. Tokioka (1950) has compared the size and form of the
cye and of the inner pigment spot, and has established several proportions that might be useful for identification of poorly preserved specimens. Net0 (1961) also regards the eye characters as useful for species
determination. Similar work has been done by Fraser (1952) on
chaetognaths in Scottish waters ; although feeling some doubt as to the
usefulness of the method, he said that in some instances it can be used to
differentiate certain species. Also Furnestin (1954) has studied the eyes of
cleveii species of the eastern Atlantic. She has stressed that the generally
well-defined limits of the central region of the eye are constant in any
one species and may constitute a good classificatory character.
The same holds for the pigmented septa which separate the visual
cupolae : their arrangement varies very little within one species while