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Class Solenogastres

Introduction:
The Solenogastres are a small class of shell-less marine molluscs, which live on the surface of mud or on cnidarians which form their principal food source. They do not normally burrow. They are regarded as being more closely related to the shell-bearing molluscs than to the Caudofoveata, with which they were once grouped in the Aplacophora.
They are sluggish animals and may remain immobile for long periods, but they can move along the surface of a secreted mucous thread by means of ciliary activity in the ventral groove. There appears to be no muscular activity involved in locomotion.
Solenogastres are hermaphrodite but it is generally held that they normally copulate and cross-fertilise. The larval development is partially known for a few species only, but it is presumed to include a planktonic phase.

Morphology:
The Solenogastres are generally small (less than 50 mm long) animals with a bilaterally symmetrical body that may be either long and slender or short and relatively broad. The body lacks a definite head or other subdivision and often careful examination is required to distinguish anterior and posterior ends. A mid-dorsal longitudinal keel or crest is often present. The ventral surface has a median longitudinal groove, the pedal groove, which terminates anteriorally in a concavity, the pedal pit, and posterially runs into the anus: this groove contains a central ridge which is considered to represent the molluscan foot. The mouth is generally a subterminal, antero-ventral vertical slit which leads into a large cavity, the vestibule or atrium. The anus opens into a terminal posterior cavity that is a cloaca. Bi-pectinate ctenidia are never present, but in the Neomeniidae, the cloaca contains secondary respiratory structures composed of lamellae, plicae or papillae.
The dorsal, lateral and greater part of the ventral surfaces are covered by a cuticle with a complex structure and, in some groups, epidermal papillae are found. These papillae are long stalks of cellular epidermis, which ascend from the epidermis through the cuticle to terminate at the surface in a spherical, vacuolated vesicle. The cuticle in all Solenogastres in the North Sea bears one or several layers of calcareous spicula, which are mainly very small. They may be set vertically but are usually backward pointing and make only a small angle with the cuticle surface. The spicula may be hollow or solid in construction and many are channel-shaped. In some species, special spicules may be found associated with the cloaca and are presumed to be copulatory in function.
The pedal groove and pedal pit are devoid of spicules and epidermal papillae but the pedal groove may have accessory folds on each side of the pedal groove proper. A heavily ciliated epidermis lines the pedal pit and both the pedal pit and the pedal groove are supplied with numerous secretory glands, which produce mucus. Some species have a radula but other species lack this structure.

Source: Jones and Baxter, 1987.