Monoplacophora

Living representatives of this Class were not discovered until 1952, although Paleozoic fossil monoplacophorans had been known for some time. At present, 11 species are known. Most live at great depths and all are marine. Monoplacophorans are small and have a single, caplike shell, giving them a limpet-like appearance. A number of their organs (nephridia, heart, etc.) are repeated serially, making them resemble metamerically-arranged species such as annelids and arthropods. Whether this resemblance indicates a close relationship between monoplacophorans and phyla exhibiting true metamerism is an open question.












Neopilina galatheae, top (dorsal) view. Neopilina is peculiar because of the replication of various of its organs and organ systems, reminiscent of metameric animals. The class Monoplacophora is well known as fossils, and until 1952 all of its members were believed to have been extinct since the Devonian period, about 350 million years ago. [This shell, relatively thin, was damaged while being dredged.]












Neopilina galatheae, bottom (ventral) view. The oval, flat foot is characteristic of the classes Monoplacophora, Polyplacophora and Gastropoda. The foot is bordered on each (left and right) sides by five gills.












Neopilina galatheae, side (lateral) view. The anterior end, denoted by the slightly coiled apex, is on the right.


One reason for having evaded detection for so long is that they are generally found in the deep ocean. Finding them has been quite a boon to malacologists however, as monoplacophorans are often thought to be among the most primitive of molluscs. Indeed, many researchers believe that monoplacophoran-like ancestors gave rise to the rest of Mollusca.

Modern systematic research has borne out the idea of Monoplacophora being the basal member of the Mollusca clade. Their morphology then, proves to be remarkably important in understanding what the first molluscs may have looked like, as well as how the other major groups such as bivalves and gastropods may have evolved.

As only a few species of living monoplacophorans are known, and all being somewhat similar, much of our knowledge of the group comes from fossils.

Fossil record
Monoplacophorans are the first undoubted molluscs, being found in rocks from the earliest Cambrian. The fossil record indicates that the group was quite diverse during the Paleozoic.

Recent monoplacophorans form a distinct clade, and their similarities and differences with the other extant molluscan groups are easily recognized. There is little question that some Paleozoic taxa are also members of this clade. However, the characters that distinguish some Paleozoic monoplacophorans from the torted gastropods and vice versa are open to alternative interpretations and the relationships of several major groups of early-shelled molluscs have therefore been the subject of much debate.

Life history & ecology
Monoplacophorans are found on both soft bottoms and hard substrates on the continental shelf and seamounts, generally in the very deep sea. However, some Paleozoic taxa are associated with relatively shallow water faunas (greater than 100 m), indicating that their relegation to the deep sea is a more recent phenomenon.

Unfortunately, there have thus far been no developmental studies done on monoplacophorans. Indeed, most of our knowledge about Monoplacophora comes from the first description of Neopilina galathaea by Lemche and Wingstrand in 1959.

More on morphology
Monoplacophorans are small and limpet-like, having a single, cap-like shell. Some organs (kidneys, heart, gills) are repeated serially, giving rise to the now falsified hypothesis that they may have a close relationship with segmented organisms such as annelids and arthropods.

In fossil monoplacophorans, the aperture (shell opening) varies in shape from almost circular to pear-shaped. Shell height is also variable and ranges from relatively flat to tall. The monoplacophoran animal has a poorly defined head with an elaborate mouth structure on the ventral surface. The mouth is typically surround by a V-shaped, thickened anterior lip and post-oral tentacles; post-oral tentacles come in a variety of morphologies and configurations. Below the head lies the semi-circular foot. In the pallial groove, between the lateral sides of the foot and the ventral mantle edge, are found five or six pairs of gills (there are fewer in very tiny taxa).







































You can download many scinece journals of Monoplacophora from this link:

http://www.zmuc.dk/inverweb/Galathea/Pdf_filer/Volume_16/galathea-vol.16-pp_007-094.pdf

http://www.mapress.com/zootaxa/2008/f/zt01866p222.pdf

http://www.zmuc.dk/InverWeb/Galathea/Pdf_filer/Volume_16/galathea-vol.16-pp_095-098.pdf

http://www.pfeil-verlag.de/04biol/pdf/spix29_3_21.pdf

http://www.auburn.edu/academic/science_math/cosam/departments/biology/faculty/webpages/zzhalanych/Pub.pdfs/Passamaneck2004a.pdf

Aplacophora

Aplacophorans are small, cylindrical, worm-like, and usually less than 5 cm long, but can range from 1 mm to 30 cm. Like other mollusks, it has no outer shell, but the epidermis secretes calcareous spicules or scales which are embedded in dorsal mantle. These spicules give the aplacophorans a sheen. Chaetoderms have a scaly appearance. All aplacophorans have a simple mantle cavity.

The radula is not ribbon-like as in other mollusks, but is an expansion of the foregut epithelium. The teeth of the radula may be in simple plates in transverse rows, up to 50 rows with 24 teeth per row. (Barnes, 1987; Brusca and Brusca, 2003; Ponder et al., 2000)

Phylum: Mollusca

Class: Aplacophora

Number of families: 30

Thumbnail description
Vermiform (worm-shaped) marine mollusks lacking shells and living in the zone between the seashore and the edge of the continental shelf

Evolution and systematics

The class Aplacophora contains two subclasses: Neomeniomorpha (also called Solenogastres) and Chaetodermomorpha (also called Caudofoveata). Most neomenioids creep by means of a narrow foot with a ventral groove that begins as a pedal pit toward the front of the animal. They have a sensory vestibule above the mouth; a single midgut organ combining stomach and digestive gland; and serial sets of muscle bands running along their sides and lower surface. Neomenioids are simultaneous hermaphrodites; they also lack ctenidia in their mantle cavities. A ctenidium is a finger-shaped or comblike structure that functions in respiration. The subclass of Neomeniomorpha comprises three orders (Pholidoskepia, Neomeniomorpha, and Cavibelonia); 24 families; 75 genera; and fewer than 250 species. The subclass Chaetodermomorpha contains six families and 15 genera. Aplacophorans in this subclass have a midgut separated into a stomach and a digestive organ, and one pair of ctenidia in their mantle cavities.

It is uncertain to what extent aplacophorans are specialized and to what extent they are primitive mollusks, but there is no evidence that they ever had shells. Their specialized features include the reduction or loss of the foot; the absence of a shell; sometimes the lack of a radula (a specialized organ unique to mollusks that allows them to scrape food from the ocean floor); and modifications of the nephridia (simple organs for excreting wastes) in certain genera to form accessory sexual organs. The possession of well-defined cerebral and pleural ganglia (groups of nerve cells) indicates that the Aplacophora are more advanced than the Polyplacophora mollusks in this respect at least. It seems probable that aplacophorans represent a secondary simplification of an ancestral form. If this hypothesis is accurate, however, the form and location of the mantle cavity in present-day aplacophorans tells researchers little about the condition of the remote ancestor. Aplacophorans have little in common with chitons (small armor-plated mollusks), although in the past the two groups were placed together in the class Amphineura.

Physical characteristics

Aplacophorans, which are also called solenogasters, are worm-shaped mollusks covered with spicules or sharp needle-like projections. The body shape varies from almost spherical to elongated and slender. These mollusks are usually less than 2 in (5 cm) in length, but adult individuals may vary from 0.039–0.078 in (1–2 mm) to 3.9 in (10 cm) or more in length.

The exterior of an aplacophoran may be spiny, smooth, or rough. The head is poorly developed, and the typical mollusk shell and foot are absent. The exoskeleton is represented only by a cuticular (horny secreted) layer that bears spicules in a variety of forms. The spicules and integument (covering) together form a character that links genera or families in this class to one another. Most aplacophorans have some specialized spicules at the entrance to the mantle cavity; these are presumably used in copulation. The cuticle and epidermis may be either thick or thin relative to the size of the species: a thick cuticle may occur together with a thin epidermis; a thin cuticle may occur with a thick epidermis; or they may be the same thickness. Glandular cells on the epidermis known as papillae may have either long stalks or no stalks at all.

Aplacophorans have a midventral longitudinal groove containing one or more ridges, which are similar in structure to the foot of other mollusks. The mantle covers the upper surface, the sides, and the greater part of the lower surface of the animal. A large gland that secretes mucus opens into the groove toward the front.

The mouth at the front of the animal opens into a muscular pharynx lined by a thick cuticle. The pharynx typically receives the products of one or two pairs of salivary glands and the radula sac. Some genera lack salivary glands. Neomenioid species creep by ciliary action of the "foot" along a sticky track of mucus produced from the ciliated, eversible pedal pit at the anterior end of the pedal groove. Both the pedal groove and the pedal pit are supplied by many mucus-secreting glands. The radula is highly variable in form. It is situated where the pharynx joins the midgut unless an esophagus is present; it may have two teeth per row, one tooth per row, or many teeth per row. The radula is lacking in 20% of known species.

The posterior end of the body contains a cavity into which one or two gametopores open as well as the anus, the copulatory spicule sacs, and the folds of respiratory tissue or papillae. The posterior cavity is believed to represent a mantle cavity. Burrowing species have a pair of gills. In Neomenia and in several other genera there is a circlet of laminar gills in the mantle cavity; in other genera, however, there are no gills.

Distribution

Aplacophorans are found in all oceans of the world; some genera have worldwide distribution. Although they have been sampled at depths ranging from 16 to 17,390 ft (5–5,300 m), the greatest diversity of species occurs at depths greater than 656 ft (200 m).

Habitat

Neomenioids live on hydroids, corals, or surface sediment. Caudofoveatans construct burrows in marine sediments, which they inhabit head downward.

Behavior

Nothing is known about the behavior of aplacophorans.

Feeding ecology and diet

Neomenioids feed on cnidarians—stony and soft corals, hydrozoans, zooantharians, or gorgonians. Some species prey only on specific cnidarians. Caudofoveatans ingest sediment or may be selective carnivores or scavengers. They feed mostly on faraminifera.

Reproductive biology

Aplacophorans are hermaphrodites and have paired gonads. Copulation probably occurs in those with the former condition, and spawning in the latter. Researchers have inferred from the presence of seminal receptacles, the structure of introsperm (sperm that never contact the water), and observation of living specimens of Epimenia australis that fertilization takes place internally.

Conservation status

No species are listed by the IUCN.

Significance to humans

Aplacophorans are used in scientific research, especially research into the evolutionary origins of mollusks.

Species accounts

Epimenia australis
Helicoradomenia juani
Spiomenia spiculata
Chaetoderma argenteum
Chevroderma turnerae
Prochaetoderma yongei

Resources

Books:

Barnes, Robert D. Invertebrate Zoology. 4th ed. Philadelphia: Saunders College Publishing, 1980.

Beesley, Pamela L., Graham J. B. Ross, and Alice Wells. Mollusca—The Southern Synthesis, Part A. Canberra: Australian Biological Resources Study, 1998.

Geise, Arthur C., and John S. Pearse. Reproduction of Marine Invertebrates. Vol. V, Mollusks: Pelecypods and Lesser Classes. London: Academic Press, 1975.

Purchon, R. D. The Biology of Mollusca. Oxford: Pergamon Press, Ltd., 1968.

Neomeniomorpha (Solenogastres): The creeping neomenioids retain a narrow foot in a ventral groove which begins anteriorly as a pedal pit. They have a papillate, a sensory supraoral vestibule, a single midgut organ combining stomach and digestive gland, serial sets of lateroventral muscle bands, they are simultaneous hermaphrodits and lack ctenidia in the mantle cavity. The photograph shows the pedal groove of Helicoradomenia sp. collected from a Pacific hydrothermal, deep-sea vent.












Chaetodermomorpha (Caudofoveata): Chaetoderms lack a foot and pedal groove. They possess either a nearly or entirely cicumoral sensory cuticular shield, the midgut is separated into a stomach and glandular digestive diverticulum, serial sets of lateroventral muscle bands lacking, they are dioecious and chaetoderms possess one pair of ctenidia in the mantle cavity. This shows Chaetoderma canadense collected from St. Margaret's, Nova Scotia, Canada.












Anatomy of the Aplacophoral;


A, Anterior. B, Posterior
AVP anteroventral radular pocket
C cuticle
CG cerebral ganglion
CM circular bodywall muscles
CPM circumpharyngeal muscle
CS copulatory spicule
DC dorsal cecum
DS dorsal sinus/aorta
DSO dorsoterminal sense organ
DSP dorsofrontal sensory pit
EP epidermal papilla
ES esophagus
F foot
G gonad
GD1 upper gametoduct
GD2 lower gametoduct
GP gametopore
GPC gonopericardial duct
HC haemocoel
LM longitudinal bodywall muscles
LN lateral nerve cord
MB membranoblast
MC mantle cavity
MG midgut
MO mouth opening
OB odontoblast
OC oral cavity
OD odontophore
OM oblique bodywall muscles
P pedal pit
PC pericardial cavity
PG pedal gland cell (greatly enlarged)
PH pharynx
R radula
RB radula bolster
RE rectum
RP respiratory papilla
SC suprarectal commissure
SG salivary gland cell
SP epidermal spicule
SR seminal receptacle
SV seminal vesicle
V ventricle
VE vestibule
VLM ventral longitudinal muscle
VN ventral nerve cord
VSG ventral salivary gland

You can download many scince journals of Aplacophora:

http://www.biologiezentrum.at/pdf_frei_remote/DENISIA_0018_0075-0140.pdf

http://biology.fullerton.edu/deernisse/pubs/Ee_Reynolds_94_MAI_chitons.pdf

http://university.uog.edu/up/micronesica/abstracts_35-36/pdfs_3536/34-miscinverts.pdf

http://www.malacological.org/meetings/archives/2003/2003_abs.pdf

http://ioc3.unesco.org/mpa/images/stories/Turnipseed%20et%20al.%202004.pdf