SOME ASPECTS O F THE BIOLOGY O F THE CHAETOGNATHS
363
caninum but Aiidreassen (1966) says that the intestine of this nematode
is not covered by cilia but has a brush border composed of microvilli ; if
the cilia are t o be found in nematodes they must' undoubtedly, as in
arthropods, be searched for in the sensory organs, and he concludes
" that cilia have not been found in nematodes ". Nevertheless, Ross
(1967) described modified cilia in sensory organs of juvenile stages of
the parasitic nematode Haemonchus contortus, and Roggen et al. (1966)
by electron microscope studies reveal the presence of true cilia in nerve
processes connected with sensory organs of another nematode (Thoracostomn cabifornicum). Also in this case there are not vibratile cilia but,
nevertheless, Rogger et al. say that the cilia which they have described
lessen the gap that exists between the nematode and the remainder of
the aschelminths. I n any case cilia are very ancient structures in
the cell, originating before the subdivision of t8he plants and animals;
in fact they have an identical structure in animals and in plants
(Lanzavecchia, 1967). For this reason they may not have an indicative
value as an evolutionary character.
Chaetognatha, according to Colosi (1967), could be classified among
the trichophores deuterostoma, while the Nematoda, like Gordiacea,
Tardigrada and Arthropoda must be considered as atricha. Other
authors (e.g. Grassi, Hyman) place Chaetognatha beside Brachiopoda,
Phoroiioidiei or Pogonophora. Hadii, as we have seen, shares this
view. D'Ancona (1966) places them among the Deuterostoma before
Pogonophora and Pterobraiichia. Salfi (1 965) also places them among
Deuterostoma but before Echinodermata, which in his classification
precede Enteropneusti and Pterobranchia. That they belong to
Deuterostoma has been questioned. Hadii, incidentally, no longer
believes in the division of animals into the two groups of Deuterostoma
and Proterostoma. According to Hyman, " the possibility that the
chaetognaths are remotely related to the Dipleurula ancestor of the
Deuterostoma, is the only justification for placing them, as done here,
among the Deuterostomia ".
Eakin and Westfall (1964) relate the chaetognaths to the Deuterostomia, i.e. Chordata and Echinodermata, on the basis of the structure
of their photoreceptive cells. The eye of Sagitta appears to be of a
ciliary type : this, according to Eakin and Westfall, should place them
among Deuterostomia since the photoreceptors of Proterostoma
(Arthropoda, Anellida, Mollusca and Plathelminthes) are of a rhabdomeric type. According to Lanzavecchia (1965) it should be kept in
mind, on the other hand, that though the cilium has a structure of the
type 9 + 0 (like vertebrates) the outer segment has a rather primitive
structure, being constituted by microvilli placed parallel with the
363
caninum but Aiidreassen (1966) says that the intestine of this nematode
is not covered by cilia but has a brush border composed of microvilli ; if
the cilia are t o be found in nematodes they must' undoubtedly, as in
arthropods, be searched for in the sensory organs, and he concludes
" that cilia have not been found in nematodes ". Nevertheless, Ross
(1967) described modified cilia in sensory organs of juvenile stages of
the parasitic nematode Haemonchus contortus, and Roggen et al. (1966)
by electron microscope studies reveal the presence of true cilia in nerve
processes connected with sensory organs of another nematode (Thoracostomn cabifornicum). Also in this case there are not vibratile cilia but,
nevertheless, Rogger et al. say that the cilia which they have described
lessen the gap that exists between the nematode and the remainder of
the aschelminths. I n any case cilia are very ancient structures in
the cell, originating before the subdivision of t8he plants and animals;
in fact they have an identical structure in animals and in plants
(Lanzavecchia, 1967). For this reason they may not have an indicative
value as an evolutionary character.
Chaetognatha, according to Colosi (1967), could be classified among
the trichophores deuterostoma, while the Nematoda, like Gordiacea,
Tardigrada and Arthropoda must be considered as atricha. Other
authors (e.g. Grassi, Hyman) place Chaetognatha beside Brachiopoda,
Phoroiioidiei or Pogonophora. Hadii, as we have seen, shares this
view. D'Ancona (1966) places them among the Deuterostoma before
Pogonophora and Pterobraiichia. Salfi (1 965) also places them among
Deuterostoma but before Echinodermata, which in his classification
precede Enteropneusti and Pterobranchia. That they belong to
Deuterostoma has been questioned. Hadii, incidentally, no longer
believes in the division of animals into the two groups of Deuterostoma
and Proterostoma. According to Hyman, " the possibility that the
chaetognaths are remotely related to the Dipleurula ancestor of the
Deuterostoma, is the only justification for placing them, as done here,
among the Deuterostomia ".
Eakin and Westfall (1964) relate the chaetognaths to the Deuterostomia, i.e. Chordata and Echinodermata, on the basis of the structure
of their photoreceptive cells. The eye of Sagitta appears to be of a
ciliary type : this, according to Eakin and Westfall, should place them
among Deuterostomia since the photoreceptors of Proterostoma
(Arthropoda, Anellida, Mollusca and Plathelminthes) are of a rhabdomeric type. According to Lanzavecchia (1965) it should be kept in
mind, on the other hand, that though the cilium has a structure of the
type 9 + 0 (like vertebrates) the outer segment has a rather primitive
structure, being constituted by microvilli placed parallel with the