Functional morphology of musculature in the acoelomate worm, Convoluta pulchra (Plathelminthes)

Convoluta pulchra is a small worm living in the surface sediment of mud fla ts where it feeds on diatoms. It is roughly teardrop in shape with a ventra l groove in which the mouth sits, and it can move in a variety of ways, rea dily distorting its body in bending, twisting, and turning motions. Fluores cently labeled probes for filamentous actin revealed the musculature in who le mounts of the worm. In the body wall, the musculature consisted of a gri d of circular, longitudinal crossover (that is, with a longitudinal orienta tion in the anterior half of the body but arcing medially to cross over to the contralateral side of the body behind the level of the mouth), and a fe w diagonal fibers. Inside the body was a strong, irregular brush of muscles originating at the rostral tip of the body and anchoring laterally and pos teriorly along the body wall, and strong dorsoventral muscles flanked the v entral groove. Two fans of muscles in the ventral and dorsal body wall reac hed posteriorly and laterally; that on the dorsal side originated at junctu res of the dorsoventral muscles with the body wall and that on the ventral body wall originated from the mouth. By their positions, certain groups of muscles could be correlated with given movements: the crossover muscles wit h some turning motions and feeding, and the inner muscles with probing and retraction motions of the rostrum and with a tuck-and-turn motion the worm used to turn itself around. Electron microscopy showed numerous maculae adh erentes junctions linking all muscle types and special junctions linking th e musculature with the epidermis. The latter myoepidermal junctions were of dimensions larger than those of maculae adherentes and contained an interl aminar material which we believe represents islands of basal matrix compara ble to basement membrane.