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 australisHelicoradomenia 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
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