0. LOWENSTEIN
210
show plastic flow. The solid otoliths are anchored to the membranous
wall of the labyrinth by systems of connective tissue strands functioning
like guy ropes. Such structures are sparingly developed in connection
with pastelike otolithic masses, In the latter case the otolithic mass of one
otolith organ can be continuous with that of another. In the elasmobranch
labyrinth such continuity has been observed between otolithic masses in
sacculus and lagena ( 0. Lowenstein, unpublished observations ) .
The general shape and layout of the labyrinth in the various classes
of chordate animals has been well described in the classic work of Retzius
(1881), and his findings are remarkably accurate considering the restricted range of techniques at his disposal. A phylogenetic line of change
leads from the myxinoids with two ampullae at either end of a dorsal
canal-like space and two ampullae at the end of two vertical canals and
a so-called macula communis via the petromyzonts with a macula already spatially subdivided into what may be interpreted as homologs of
the maculae of the gnathostome labyrinth (Lowenstein et al., 1968;
Lowenstein and Thornhill, 1971 ) to the gnathostome labyrinth with three
semicircular canals and three well-separated otolith organs, viz., the utriculus, sacculus, and lagena, each equipped with a sensory macula carrying an otolith.
The labyrinth of Myxine (Fig. 1A) lies fully enclosed in a cartilaginous
capsule. On the floor of the membranous labyrinth lies a macula consisting of sensory hair cells and supporting cells. The hair processes of the
sensory cells project into a jellylike “otolith membrane” which is loaded
with numerous large spherites of carbonate of lime. They form a continuous otolithic mass. The macula itself lies in a near vertical plane with
its anterior and posterior margins slightly curved toward the horizontal.
As will be seen below, there is ultrastructural evidence that this macula
communis contains three areas corresponding to the three maculae of the
gnathostome labyrinth. There are two roughly spherical ampullary spaces
above the macula each with a ring-shaped crista carrying an epithelium
of sensory hair cells. Their orientation within each crista makes it plausible
that the ampullae correspond basically to the anterior and posterior vertical ampullae of the gnathostomes. However, it can be shown that they
are capable of signalling horizontal angular accelerations, i.e., of playing
the role later taken over by the horizontal canal of the gnathostome
labyrinth ( Lowenstein, 1970; Lowenstein and Thornhill, 1971). The upper
cavity of the labyrinth of Myxine represents a fairly wide open space
without any special compartmentalization that could properly be described as tubular vertical canals. The VIIIth nerve is subdivided into
branches supplying the two ampullae and the various regions of the
macula.
210
show plastic flow. The solid otoliths are anchored to the membranous
wall of the labyrinth by systems of connective tissue strands functioning
like guy ropes. Such structures are sparingly developed in connection
with pastelike otolithic masses, In the latter case the otolithic mass of one
otolith organ can be continuous with that of another. In the elasmobranch
labyrinth such continuity has been observed between otolithic masses in
sacculus and lagena ( 0. Lowenstein, unpublished observations ) .
The general shape and layout of the labyrinth in the various classes
of chordate animals has been well described in the classic work of Retzius
(1881), and his findings are remarkably accurate considering the restricted range of techniques at his disposal. A phylogenetic line of change
leads from the myxinoids with two ampullae at either end of a dorsal
canal-like space and two ampullae at the end of two vertical canals and
a so-called macula communis via the petromyzonts with a macula already spatially subdivided into what may be interpreted as homologs of
the maculae of the gnathostome labyrinth (Lowenstein et al., 1968;
Lowenstein and Thornhill, 1971 ) to the gnathostome labyrinth with three
semicircular canals and three well-separated otolith organs, viz., the utriculus, sacculus, and lagena, each equipped with a sensory macula carrying an otolith.
The labyrinth of Myxine (Fig. 1A) lies fully enclosed in a cartilaginous
capsule. On the floor of the membranous labyrinth lies a macula consisting of sensory hair cells and supporting cells. The hair processes of the
sensory cells project into a jellylike “otolith membrane” which is loaded
with numerous large spherites of carbonate of lime. They form a continuous otolithic mass. The macula itself lies in a near vertical plane with
its anterior and posterior margins slightly curved toward the horizontal.
As will be seen below, there is ultrastructural evidence that this macula
communis contains three areas corresponding to the three maculae of the
gnathostome labyrinth. There are two roughly spherical ampullary spaces
above the macula each with a ring-shaped crista carrying an epithelium
of sensory hair cells. Their orientation within each crista makes it plausible
that the ampullae correspond basically to the anterior and posterior vertical ampullae of the gnathostomes. However, it can be shown that they
are capable of signalling horizontal angular accelerations, i.e., of playing
the role later taken over by the horizontal canal of the gnathostome
labyrinth ( Lowenstein, 1970; Lowenstein and Thornhill, 1971). The upper
cavity of the labyrinth of Myxine represents a fairly wide open space
without any special compartmentalization that could properly be described as tubular vertical canals. The VIIIth nerve is subdivided into
branches supplying the two ampullae and the various regions of the
macula.