SOME ASPECTS O F T H E BIOLOGY O F T H E CHAETOGNATHS
361
and the neural glands of the tunicates and the retrocerebral organ of
the chaetognaths. Scharrer (1965) while noting a superficial likeness
between the retrocerebral organ and the glomeruli that form the
neuropile of the nucleus rotundus in the tegumentum of the teleostean
mesencephalon, stated after examination of the retrocerebral organ
with the electron microscope, that the masses of microvilli of the retrocerebral organ are similar to those described in the cerebral ganglion
of the crustacean Leptodora. The occurrence of cells with microvillous
borders in the central nervous system of the representatives of two
different phyla, suggests a role related to functional requirements that
the types of animals have in common (Scharrer, 1965). Reisinger further
finds a significant likeness between the ultrastructure of the ciliary
sense organs (" neuromasten ") of the chaetognaths and the sensory cell
of the lateral line in the Anamniotes. Furthermore, the fin rays of
actinotrichian type with substance similar to elastoidin, remind one of
those of the fins of cyclostomes and fishes and are not to be found in
other invertebrates beside chaetognaths.
Like them, the
chaetognaths show a cephalic armature and a cuticular lining of the
body. The lateral fins could perhaps be considered similar to the alae
of nematodes. The musculature is placed in four longitudinal fields,
and as we have seen the adult chaetognaths have the appearance of
pseudocoelomate animals like the nematodes. Also the macroscopic
and microscopic structures of the intestine are similar. Other features
in common are the precocious differentiation of the germinal line and
the almost total inability to regenerate the amputated parts.
The hooks of
Chaetognatha, as we have seen, have a chitinous nature, while the
supposed chitin of the body-lining of nematodes is to be considered as
a secreted collagen in which keratin is perhaps present but no chitin
(Rizzoli, 1956). The nervous system in the two groups differs considerably. The muscular system has only the distribution of the
muscular bands in four longitudinal fields, similar to that of the nematodes. This disposition is, however, common, as we saw, t o annelids
also and it serves to ensure the movement by contraction of the body,
in all directions, with the minimum number of muscular bands possible.
The histological structure of the musculature of Nematoda and
Chaetognatha is quite different. The muscles of the Chaetognatha show
indeed typically striated fibres. Camatini and Lanzavecchia (1966) have
lately observed this fibre under the electron microscope. They were
able to see that within each fibre there are numerous myofibrillae with
traiisversc striae in line with cach other. Almost all of the sarcomere i
The likeness to the Nematoda is more obvious.
The differences, however, are not less important.
361
and the neural glands of the tunicates and the retrocerebral organ of
the chaetognaths. Scharrer (1965) while noting a superficial likeness
between the retrocerebral organ and the glomeruli that form the
neuropile of the nucleus rotundus in the tegumentum of the teleostean
mesencephalon, stated after examination of the retrocerebral organ
with the electron microscope, that the masses of microvilli of the retrocerebral organ are similar to those described in the cerebral ganglion
of the crustacean Leptodora. The occurrence of cells with microvillous
borders in the central nervous system of the representatives of two
different phyla, suggests a role related to functional requirements that
the types of animals have in common (Scharrer, 1965). Reisinger further
finds a significant likeness between the ultrastructure of the ciliary
sense organs (" neuromasten ") of the chaetognaths and the sensory cell
of the lateral line in the Anamniotes. Furthermore, the fin rays of
actinotrichian type with substance similar to elastoidin, remind one of
those of the fins of cyclostomes and fishes and are not to be found in
other invertebrates beside chaetognaths.
Like them, the
chaetognaths show a cephalic armature and a cuticular lining of the
body. The lateral fins could perhaps be considered similar to the alae
of nematodes. The musculature is placed in four longitudinal fields,
and as we have seen the adult chaetognaths have the appearance of
pseudocoelomate animals like the nematodes. Also the macroscopic
and microscopic structures of the intestine are similar. Other features
in common are the precocious differentiation of the germinal line and
the almost total inability to regenerate the amputated parts.
The hooks of
Chaetognatha, as we have seen, have a chitinous nature, while the
supposed chitin of the body-lining of nematodes is to be considered as
a secreted collagen in which keratin is perhaps present but no chitin
(Rizzoli, 1956). The nervous system in the two groups differs considerably. The muscular system has only the distribution of the
muscular bands in four longitudinal fields, similar to that of the nematodes. This disposition is, however, common, as we saw, t o annelids
also and it serves to ensure the movement by contraction of the body,
in all directions, with the minimum number of muscular bands possible.
The histological structure of the musculature of Nematoda and
Chaetognatha is quite different. The muscles of the Chaetognatha show
indeed typically striated fibres. Camatini and Lanzavecchia (1966) have
lately observed this fibre under the electron microscope. They were
able to see that within each fibre there are numerous myofibrillae with
traiisversc striae in line with cach other. Almost all of the sarcomere i
The likeness to the Nematoda is more obvious.
The differences, however, are not less important.