SOME ASPECTS O F THE BIOLOGY O F THE CHAETOGNATIIS
275
the multistratified nature of the epidermis and describes inter- or intrccellular nervous fibrillae which are in connection with the ventral
ganglion and form a plexus which innervates the sense organs of the
epidermis. I n several species there is t o be found a formation developed
in the region of the neck which sometimes reaches and passes the caudal
septum and has been named " collarette )' or lateral expansion of the
epidermis (Grassi, 1883). This structure is formed by big vacuolated
cells that roughly resemble the cells of the dorsal chord: this is why
Grassi called the tissue forming the collarette " chordoid tissue ". It
is specially developed in two genera, i.e. Spudella and Pterosagitta.
It is interesting to note that the maximum development of the
collarette is to be found in two species whose habitat is completely
different. Pterosagitta draco (Krohn) is a typical pelagic species and
the presence of the vacuolated cells seems t o serve the purpose of
making floating easier. On the other hand, Spadella cephaloptera is a
benthic species that lives attached, by means of special adhesive
glands, t o the leaves of phanerogams or of submerged seaweeds. The
collarette, though much less developed, is also present in other pelagic
species like Sagitta bipunctata Quoy and Gaimard or 8. serratodentata.
S. crassa also has a well-developed collarette which changes in size
according t o the variations of chlorinity and temperature. There are
four classes of development of the collarette from class A, characteristic
of 8. crassa typica, t o class D of S. crassa forma naikaiensis (Hirota,
1959 ; see also p. 348). It was supposed that the development of the
collarette of the large form of S. crassa might be significant for planktonic life (Kado and Hirota, 1957). According t o Tokioka (1965b) the
development of the collarette does not seem to imply any phylogenically
significant specialization. Originally the collarette might be useful as a
protective tissue, but when well developed it might contribute t o the
buoyancy of animals of moderate size; generally pelagic animals of
much smaller size would not need such a structure.
More or less conical masses of cells are often t o be noted on the
epidermis, in the middle of which one sees some small bristles that are
deemed to be sensory (Hertwig, 1880 ; Grassi, 1883). Two quite peculiar
tufts of bristles are to be found at the sides of the neck of Pterosagitta
Horridge (1966) and Horridge and Boulton (1967) in Spudella
cephaloptera, using the electron microscope and phase contrast microscope, were able t o observe two types of tufts along the body not
previously distinguished : the tufts of bristles which are structurally not
sense organs and the tufts of cilia which are. The bristles are seen in
life whereas the ciliary tufts are visible only in sections or in life with
araco.
275
the multistratified nature of the epidermis and describes inter- or intrccellular nervous fibrillae which are in connection with the ventral
ganglion and form a plexus which innervates the sense organs of the
epidermis. I n several species there is t o be found a formation developed
in the region of the neck which sometimes reaches and passes the caudal
septum and has been named " collarette )' or lateral expansion of the
epidermis (Grassi, 1883). This structure is formed by big vacuolated
cells that roughly resemble the cells of the dorsal chord: this is why
Grassi called the tissue forming the collarette " chordoid tissue ". It
is specially developed in two genera, i.e. Spudella and Pterosagitta.
It is interesting to note that the maximum development of the
collarette is to be found in two species whose habitat is completely
different. Pterosagitta draco (Krohn) is a typical pelagic species and
the presence of the vacuolated cells seems t o serve the purpose of
making floating easier. On the other hand, Spadella cephaloptera is a
benthic species that lives attached, by means of special adhesive
glands, t o the leaves of phanerogams or of submerged seaweeds. The
collarette, though much less developed, is also present in other pelagic
species like Sagitta bipunctata Quoy and Gaimard or 8. serratodentata.
S. crassa also has a well-developed collarette which changes in size
according t o the variations of chlorinity and temperature. There are
four classes of development of the collarette from class A, characteristic
of 8. crassa typica, t o class D of S. crassa forma naikaiensis (Hirota,
1959 ; see also p. 348). It was supposed that the development of the
collarette of the large form of S. crassa might be significant for planktonic life (Kado and Hirota, 1957). According t o Tokioka (1965b) the
development of the collarette does not seem to imply any phylogenically
significant specialization. Originally the collarette might be useful as a
protective tissue, but when well developed it might contribute t o the
buoyancy of animals of moderate size; generally pelagic animals of
much smaller size would not need such a structure.
More or less conical masses of cells are often t o be noted on the
epidermis, in the middle of which one sees some small bristles that are
deemed to be sensory (Hertwig, 1880 ; Grassi, 1883). Two quite peculiar
tufts of bristles are to be found at the sides of the neck of Pterosagitta
Horridge (1966) and Horridge and Boulton (1967) in Spudella
cephaloptera, using the electron microscope and phase contrast microscope, were able t o observe two types of tufts along the body not
previously distinguished : the tufts of bristles which are structurally not
sense organs and the tufts of cilia which are. The bristles are seen in
life whereas the ciliary tufts are visible only in sections or in life with
araco.