CLASS APLACOPHORA

(Summary from University of Florida, Zoology 6207, Spring 1982)

Beware: Scanned document.

I. Historical account

A.   Chaetoderma discovered in 1841 by Loven, considered as a holothurian.

B.    Graff (1875/1877) described the anatomy of Chaetoderma and Neomenia and concluded they were Molluscs.

C.    Von Ihering (1876) united these animals (he called Aplacophora) and Chitons (Polyplacophora) in the group Vermes Amphineura.

D.    The name solenogasters dates from Gegenbaur (1878), but the characteristic on which the name is based is not present in all members of the group.

E.    Groups were done and undone, as the molluscan nature of solenogasters was questioned as late as 1925(Thiele). Since then they were accepted as a molluscan class by most authors, but often together with Polyplacophora in Amphineura.


II.    Definition of the class


A.    Molluscan characters coelomate Bilateria with little or no metamerism, a soft body, a pharynx with a radula, gills, a heart, an open circulatory system.

B.    Aplacophoran characters no shell (mantle), foot, nephridia. Straight digestive tract, a pair of coelomoducts serving as gonoducts.


III.    External features

A.    Size often less than 5 cm, though one species reaches 30 cm.

B.    Coloration pale, with a silvery sheen due to the spicules.

C.    Body vermiform, cylindroid with even or varying thickness, often short or plump.

D.    Head poorly differentiated; no definite divisions of body.

E.    Absence of shell, mantle or foot.

F.    Integument with several layers of embedded calcareous spicules, generally inconspicuous (spicules lack in 3 species).

C.    Ventrally in-rolled body margins. Ventral groove in non-burrowing species. This "pedal groove" terminates in a pedal pit anteriorly and into the anus posteriorly. It contains one or more ridges.

H.    Mouth a subterminal slit, very expansible and often preceded by a large vestibule. A fold may separate vestibule from mouth. Often an oral shield with no spicules.
I.    Posterior end with a mantle cavity (cloaca) into which the subterminal anus and a pair of coelomoducts empty. A pair of gills in burrowing species, or several folds with respiratory function.


IV.    Internal anatomy

A.    Body wall:

1.    Cuticle: homogeneous, more or less thick, continuous in some species with distal ends of epidermal cells.

2.    Epidermis: one layer of cuboidal to columnar cells, including cuticle-secreting cells, and large secretive cells producing spicules or epidermal papillae with excretory function.

3.    Spicules: microscopic, calcareous, in radial or tangential position; the number of layers depends on the thickness of the cuticle. Variety of shapes. Associated with particular body areas.

4.    Muscle layer directly on the connective tissue, separated from the epidermis by a basilar membrane.

5.    Sometimes a dermis with nerves, muscles, blood sinuses.

6.    Oral shield: no spicules, no papillae. Columnar epidermis, sensory cells with ganglionated nerves, multicellular glands.

7.    Pedal groove and pedal pit also without spicules and papillae. Bear multicellular mucous glands. Median fold has connective and muscle tissue, a tall ciliated epidermis and sensory cells. Outer folds are flattened, bear little connective tissue. Striated cuticle on inner side. Pedal pit has heavily ciliated epidermis.

8.    Some authors think that the wall of the pedal groove represents the mantle.


B. Muscles :    variable pattern. Generally:

1.    One outer circular layer.

2.    Two middle diagonal layers, at right angles of each other.

3.    One inner longitudinal layer.

4.    Musculature usually poorly developed, but stronger ventrally by addition of two longitudinal bundles ceasing at the pedal groove.

5.    Smooth muscles.

C.    Body cavity

1.    Space between wall and digestive tract contains strands of connective tissue. Interstices filled with blood.

2.    Pericardium a true coelomic cavity and the gonoducts are its coelomoducts.

D.    Digestive tract

1.    Mouth a vertical slit often bordered by a ciliary labial ridge.

2.    Vestibule, often separated from the foregut by a fold. Filled with branched papillae with sensory function. Ciliated ridges also occur.

3.    Foregut may be lined with cuticle, and has a coat of circular and sometimes longitudinal muscles. The foregut may be eversible and associated with retractor muscles.

4.    Salivary glands: small, uni- or multicellular pharyngeal glands, dorsal salivary glands with clusters of large multicellular follicles, and ventral salivary glands, long and tubular.

5.    Radula in the posterior foregut. In contrast to Gastropoda, teeth are mounted on a basal expansion of the epithelium. Up to 50x24 teeth of various shapes. Anteriorly worn teeth are discarded. Posteriorly, new teeth are secreted in the radular sac. Bolsters of connective and muscular tissue, or chondroid cells, support the radula. Special radular muscles and circular muscles of the foregut wall.

5.    Subradular organ : probable sensory function.

6.    Midgut: straight, with a dorsal caecum, or a "liver". Glandular epithelium of columnar and clavifortn cells.

7.    Hindgut: narrower, usually short. Ciliated epithelium and some muscle tissue.

8.    Anal chamber or a true cloaca: cuboidal or columnar, ciliated epithelium. Thin cuticle. Preanal glands, sometimes copulatory spicules, retractors, and protractors.

E.    Circulatory and respiratory organs.

1.    Open circulatory system.

2.    Heart in pericardium: ventricle, auricle, a pair of caeca invaginated from the pericardial wall. Circulation diffuses from the dorsal sinus, then into the ventral sinus. Both are not definite structures.

3.    Two types of blood cells: a rounded or elliptical, flattened type, and a smaller, amoeboid type.

4.    Respiration by contraction of gills or gill folds.

F.    Excretory organs.

1.    Only glands: old club glands become vacuolated and their content is poured into the gut lumen.

2.    Amoebocytes discharge into epidermal papillae. Breaking of the end vesicles follows.

G.    Nervous system and sensory organs

1.    Large supraenteric ganglia, compact or bilobed.

2.    One to five pairs of cerebroid nerves with ganglionic bases.

3.    Two buccal ganglia with ventral commissures forming one to three rings. Pericaryons are assembled in ganglia.

4.    A variety of accessory ganglia according to the species.

5.    Often lateral and pedal ganglia with respective nerve chords. Nerves connect from terminal ganglia to the dorsocaudal organ, gills and retractors.

6.    Sense organs: sensory pits on oral shield and dorsocaudal organ, vestibular papillae, ciliated ridges of mouth and vestibule, subradular organ, gills or folds, pedal groove folds.

H.    Reproductive system

1.    Dioecious or hermaphroditic.

2.    Single or paired gonad: dorsal, tubular, diverticulated. In hermaphroditic species, ovaries on the medial wall, testes on the lateral wall.

3.    A gonopericardial canal opens into pericardium. Sex cells pass through the pericardial cavity, then through the pair of gonoducts, the seminal vesicles. Then the gonoducts bend posteriorly. Seminal receptacle(s), shell glands lateral to cloaca  (probably secrete egg capsule).

V.    Reproduction

A.    Copulation occurs in hermaphroditic forms.

B.    Spawning in dioecious forms: flattened egg bands with mucus issue from the cloaca.

C.    Brooding in cloacal pouches to the trocophore stage has been recorded.

VI.    Embryology

A.    Comparable to primitive Bivalves, but different modes are found.

B.   Nemcatomenia: Cleavage produces unequal blastomeres: 4 macro- and 28 micromeres. Gastrulation with invagination of macromeres and

epiboly of micromeres, forming a large blastopore. Development of cilia in the free larva: a tuft in the apical region, middle cilia, caudary cilia. The caudal bud elongates and secretes temporary spicules.

C.   Rhopalamenia: embolic gastrulation. Unequal cleavage. Embryo completely ciliated. Regionation less distinct. Internal mass : the apical part forms the entoderm, the remaining part proliferates to form a middle fold (then proctodaeum and ectoderm), and lateral folds (then mesoderm). The stomodaeum is formed by invagination.

D. Halomenia: equal cleavage. Entoderm originates by delamination of interior cells. Nervous tissue proliferates from surface cells. Larva has no apical tuft or sense organs. It is brooded in cloacal pouch, doesn't swim.

E. Epimenia: spiral cleavage. Gastrulation produces a trochophore with 3 regions. It elongates into a vermiform shape and undergoes a direct development, shedding its ciliary girdles and apical region. The trochophore lacks nephridia, in contrast to annelids. Stomodaeal and proctodaeal invaginations form the foregut, hindgut. Cerebral ganglia and nerves are formed from the ectoderm.

F.   Neomenia: unequal cleavage. Embolic gastrulation. Ingression of cells from the pseudoblastopore, forms ento- and mesoderm. The ectoderm forms stomodaeum, proctodaeum, and epidermis. The larva is similar to those described above, with apical tuft and ciliary girdles. Ectodermal bud elongates to form the trunk epidermis and nervous tissues. The endoderm develops into the midgut.

VII.    Ecology and physiology

A.    Marine, benthic animals, found throughout the oceans of the world, at depths of 18 to 9000 m.

B.    Very sluggish. Burrow in mud bottoms or creep on hydroids, corals, seaweeds, with which they are specifically associated.

C.    Burrowers are selective carnivores and scavengers. Other forms feed on coelenterates with which they live. The oral shield is associated with food selection and intake.

D.    Burrowers orient vertically with the anterior end down and expanded gills at the burrow mouth.

VIII. Classification:    130 sp in 2 orders.

A.    Order Neomenioidea: 100 sp. Pedal groove, respiratory folds, dioecious. Families Lepidomeniidae, Neomeniidae, Proneomeniidae.

B.    Order Chaetodermatoidae: 30 sp. Pedal groove lost. Pedal pit, vestibular papillae. No ventral salivary gland. Gills. Hermaphroditic. Fused gonads. Long posterior midgut caecum. Specialized radula. Family Chaetodermatiidae.

IX.    Phylogeny

A.    To what extent Aplacophora are specialized or primitive molluscs is uncertain. Some features (nervous system) show affinity with Polyplacophora..

B.    One hypothesis places them as a preliminary stage in the evolution of molluscs. They lack typical features such as shell, mantle, foot, nephridia. Gonoducts issue from the pericardium. Aplacophora and Polyplacophora diverged early in molluscan history, before a shell was formed.

C.    Second hypothesis: They lost their shell secondarily. Ganglia are evolved features.

X.    References

Barnes R.D., 1969 : Invertebrate zoology, 2nd Ed. Saunders Co, Philadelphia. p 375, p 431.

Gosner K.L., 1971 : Guide to the identification of marine and estuarine invertebrates. Cape Hatteras to the Bay of Fundy. pp252-253. Wiley Interscience, NY.

Grassê P.P., 1959 : Précis de zoobogie. Masson, Paris.

Hyman L.H., 1959: The invertebrates, Vol. VI: Molluscs-1. pp 1-70. McGraw-Hill, NY.

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