Neopilina
Encyclopedia
Neopilina is a highly derived genus of modern monoplacophoran.
However, molecular methods show that they fall within the polyplacophoran clade, and fossil and morphological data show that they are rather derived and bear very little resemblance to an 'ancestral mollusc'.
Its pair of preoral tentacles are considered homologous to those of gastropods; like prosobranch gastropod tentacles, their nerves connect to the cerebal ganglia of the brain. The post-oral tentacles are equated with bivalves' labial flaps, cephalopod
s' arms, and scaphopods' captacula.
Cuticular hardenings around the mouth of the organism are considered to be jaw-like and very not far removed from the beaks of cephalopods, or the jaws of many gastropods.
The presence of a single shell prompts comparisons to the cephalopod Nautilus
, but besides its bilateral symmetry and direction of coiling, there is not a clear equivalence; Nautlius shell is notably different in the possession of septa (and thus a sphuncle). It bears a similar degree of similarity to most other mollusc groups, leading to speculation that it may reflect a relatively unchanged ancestral mollusc. The shell itself is aragonitic, consisting mainly of a prismatic layer, lined with nacre
.
The organism bears five pairs of ctenidia, unusually for molluscs; the rear two are homologous to the two pairs in Nautilus. This is unlike the polyplacophora, who have a number of pairs of ctenidia, but this number varies and is not related to the number of their body 'segments'.
The foot and pallial groove are very difficult indeed to discriminate from the polyplacophora, supporting its placement in this group by molecular methods
Its radula is not unlike that of the polyplacophora; notably, its fifth tooth is modified to be comb-like.
However, molecular methods show that they fall within the polyplacophoran clade, and fossil and morphological data show that they are rather derived and bear very little resemblance to an 'ancestral mollusc'.
Anatomy
Its anatomy led researchers to believe that the cephalopods evolved from the monoplacophora.Its pair of preoral tentacles are considered homologous to those of gastropods; like prosobranch gastropod tentacles, their nerves connect to the cerebal ganglia of the brain. The post-oral tentacles are equated with bivalves' labial flaps, cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s' arms, and scaphopods' captacula.
Cuticular hardenings around the mouth of the organism are considered to be jaw-like and very not far removed from the beaks of cephalopods, or the jaws of many gastropods.
The presence of a single shell prompts comparisons to the cephalopod Nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
, but besides its bilateral symmetry and direction of coiling, there is not a clear equivalence; Nautlius shell is notably different in the possession of septa (and thus a sphuncle). It bears a similar degree of similarity to most other mollusc groups, leading to speculation that it may reflect a relatively unchanged ancestral mollusc. The shell itself is aragonitic, consisting mainly of a prismatic layer, lined with nacre
Nacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
.
The organism bears five pairs of ctenidia, unusually for molluscs; the rear two are homologous to the two pairs in Nautilus. This is unlike the polyplacophora, who have a number of pairs of ctenidia, but this number varies and is not related to the number of their body 'segments'.
The foot and pallial groove are very difficult indeed to discriminate from the polyplacophora, supporting its placement in this group by molecular methods
Its radula is not unlike that of the polyplacophora; notably, its fifth tooth is modified to be comb-like.