134
The lancelet is a typical representative of the subphylum Cephalochordata, most
closely related to vertebrates. The animal is an interesting model organism because
it has neither an ear nor organs that resemble the lateral line organs (Fritzsch 1996 ;
Lacalli 2004 ). In contrast to lancelet, all craniate vertebrates possess so called
angular and linear acceleration sensors. These two discrete sensory systems are the
essential components of the ear. Such primitive vertebrates as lampreys and hagfi sh
possess “ the gravistatic receptor is a single macula covered with otoconia ” (Beisel
et al. 2005 ; see also Lewis et al. 1985 ). It is suggested that that vertebrate ear evolution is primarily the diversifi cation of a statocyst (Beisel et al. 2005 ), the structure
that is known in invertebrates. Defi nitively, the vertebrate’s ear that consists of three
different organs (utricle, saccule, lagena), represents more complex sensory system
than statocyst (Lewis et al. 1985 ; Retzius 1884 ).
Being on the organ level of organization, I wish briefl y to describe the role of
some of them with respect to otoconia. For example, in Elasmobranchii, the lagena
is part of the vestibular system that is responsible for orientation with respect to the
gravitation force, as well as for hearing. Anatomically, the lagenar macula appears
fi rst in the posterior part of the sacculus of this fi sh group (Khorevin 2008 ). Also,
the saccular macula found within the sacculus of the fi sh inner ear is, probably, to
be responsible for sound detection in elasmobranchs (Corwin 1981 ). According
to Popper et al. ( 2003 ), “ this macula is hypothesized to be stimulated in response to
sound waves, as a result of a smaller displacement of the dense overlying otoconial
mass relative to the displacement of the fi sh’s body, ” (Mills et al. 2011 ; see for more
information Popper et al. 2003 ). Under “otoconial mass” we mean some kind of
gelatinous matrix that bind together the assemblages of otoconia, calcite, aragonite,
vaterite, or calcium carbonate monohydrate crystals. The elasmobranch otoconial
mass is analogous to the otolith (see below) of other fi sh.
In amphibians, the lagena is responsible for vestibular and auditory functions.
This structure is localized in the posterior part of the sacculus. The lagena in birds
is crucial for their navigation. Lagenar otoconia in birds, including sea birds
species, possess magnetic properties and, correspondingly, are supposed to be capable of orienting within the magnetic fi eld of the Earth. Mammals (with exception
of Monotremata like platypus and echidna), have no lagena as an “ independent
endorgan ” (Khorevin 2008 ).
Otolithic membrane of utricles, saccules, and lagena represent the next level of
organization of the otoconia-containing organs and formations. So called otoconial
comples is the specialized extremely dense matrix that is localized only within the
utricle and saccule. It “ overlies the sensory epithelium and provides inertial mass to
generate shearing forces essential for the mechanoreceptors to sense gravity and
linear acceleration, ” (Lundberg et al. 2006 ). Otolithic membrane in various otolithic
organs in many animals was found to differ by shape, size, structure, and composition
of otoconia (see for review Lychakov 2004 ). Otolithic membrane of utricle of
amphibians and reptiles appears as a thin plate of non-uniform structure. Otolithic
apparatus in saccule is a large cobble-stone-like conglomerate of otoconia. Otolithic
membrane of lagena looks like a bent plate and is poorly differentiated in amphibians,
but well differentiated in reptiles. Thus, transition of vertebrates to the earth surface
3 Biocomposites and Mineralized Tissues
The lancelet is a typical representative of the subphylum Cephalochordata, most
closely related to vertebrates. The animal is an interesting model organism because
it has neither an ear nor organs that resemble the lateral line organs (Fritzsch 1996 ;
Lacalli 2004 ). In contrast to lancelet, all craniate vertebrates possess so called
angular and linear acceleration sensors. These two discrete sensory systems are the
essential components of the ear. Such primitive vertebrates as lampreys and hagfi sh
possess “ the gravistatic receptor is a single macula covered with otoconia ” (Beisel
et al. 2005 ; see also Lewis et al. 1985 ). It is suggested that that vertebrate ear evolution is primarily the diversifi cation of a statocyst (Beisel et al. 2005 ), the structure
that is known in invertebrates. Defi nitively, the vertebrate’s ear that consists of three
different organs (utricle, saccule, lagena), represents more complex sensory system
than statocyst (Lewis et al. 1985 ; Retzius 1884 ).
Being on the organ level of organization, I wish briefl y to describe the role of
some of them with respect to otoconia. For example, in Elasmobranchii, the lagena
is part of the vestibular system that is responsible for orientation with respect to the
gravitation force, as well as for hearing. Anatomically, the lagenar macula appears
fi rst in the posterior part of the sacculus of this fi sh group (Khorevin 2008 ). Also,
the saccular macula found within the sacculus of the fi sh inner ear is, probably, to
be responsible for sound detection in elasmobranchs (Corwin 1981 ). According
to Popper et al. ( 2003 ), “ this macula is hypothesized to be stimulated in response to
sound waves, as a result of a smaller displacement of the dense overlying otoconial
mass relative to the displacement of the fi sh’s body, ” (Mills et al. 2011 ; see for more
information Popper et al. 2003 ). Under “otoconial mass” we mean some kind of
gelatinous matrix that bind together the assemblages of otoconia, calcite, aragonite,
vaterite, or calcium carbonate monohydrate crystals. The elasmobranch otoconial
mass is analogous to the otolith (see below) of other fi sh.
In amphibians, the lagena is responsible for vestibular and auditory functions.
This structure is localized in the posterior part of the sacculus. The lagena in birds
is crucial for their navigation. Lagenar otoconia in birds, including sea birds
species, possess magnetic properties and, correspondingly, are supposed to be capable of orienting within the magnetic fi eld of the Earth. Mammals (with exception
of Monotremata like platypus and echidna), have no lagena as an “ independent
endorgan ” (Khorevin 2008 ).
Otolithic membrane of utricles, saccules, and lagena represent the next level of
organization of the otoconia-containing organs and formations. So called otoconial
comples is the specialized extremely dense matrix that is localized only within the
utricle and saccule. It “ overlies the sensory epithelium and provides inertial mass to
generate shearing forces essential for the mechanoreceptors to sense gravity and
linear acceleration, ” (Lundberg et al. 2006 ). Otolithic membrane in various otolithic
organs in many animals was found to differ by shape, size, structure, and composition
of otoconia (see for review Lychakov 2004 ). Otolithic membrane of utricle of
amphibians and reptiles appears as a thin plate of non-uniform structure. Otolithic
apparatus in saccule is a large cobble-stone-like conglomerate of otoconia. Otolithic
membrane of lagena looks like a bent plate and is poorly differentiated in amphibians,
but well differentiated in reptiles. Thus, transition of vertebrates to the earth surface
3 Biocomposites and Mineralized Tissues