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

Abstract

Convoluta pulchra is a small worm living in the surface sediment of mud flats where it feeds on diatoms. It is roughly teardrop in shape with a ventral groove in which the mouth sits, and it can move in a variety of ways, readily distorting its body in bending, twisting, and turning motions. Fluorescently labeled probes for filamentous actin revealed the musculature in whole mounts of the worm. In the body wall, the musculature consisted of a grid of circular, longitudinal crossover (that is, with a longitudinal orientation 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 few diagonal fibers. Inside the body was a strong, irregular brush of muscles originating at the rostral tip of the body and anchoring laterally and posteriorly along the body wall, and strong dorsoventral muscles flanked the ventral groove. Two fans of muscles in the ventral and dorsal body wall reached posteriorly and laterally; that on the dorsal side originated at junctures 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 with 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 adherentes junctions linking all muscle types and special junctions linking the musculature with the epidermis. The latter myoepidermal junctions were of dimensions larger than those of maculae adherentes and contained an interlaminar material which we believe represents islands of basal matrix comparable to basement membrane.

A cultivable acoel species from the Mediterranean, Aphanostoma pisae sp. nov. (Acoela, Acoelomorpha)

“…3) is generally similar to that of other isodiametrids (Tyler & Rieger, 1999;Ladurner & Rieger, 2000;Hooge, 2001). Unlike Isodiametra pulchra, there are no ventral diagonal fibers that terminate at the midline before crossing over in A. pisae (Tyler & Rieger, 1999;Ladurner & Rieger, 2000). Hooge (2001) Hooge & Tyler, 2003; different from all aforementioned species, A. pisae's longitudinal ventral muscle fibers from mouth to posteriad stop short at the anterior part of the bursa, instead of reaching the posterior end.…”

“…In A. pisae, the body wall musculature (Fig. 3) is generally similar to that of other isodiametrids (Tyler & Rieger, 1999;Ladurner & Rieger, 2000;Hooge, 2001). Unlike Isodiametra pulchra, there are no ventral diagonal fibers that terminate at the midline before crossing over in A. pisae (Tyler & Rieger, 1999;Ladurner & Rieger, 2000).…”

Chiton myogenesis: Perspectives for the development and evolution of larval and adult muscle systems in molluscs

“…Thus, the molluscan dorsoventral musculature can be regarded as apomorphic for the phylum (e.g., Salvini-Plawen, 1980;Haszprunar, 1988;Haszprunar and Wanninger, 2000). Platyhelminthes, however, also express dorsoventral muscles different from that of molluscs and distinct from the typical worm-like body-wall musculature (Tyler and Rieger, 1999). The body-wall musculature of worm-shaped groups like annelids, platyhelminths, or nemerteans mainly consists of three layers of ring, diagonal, and longitudinal muscles (e.g., Rieger et al, 1994;Reiter et al, 1996;Hooge and Tyler, 1999).…”

An option space for early neural evolution

“…The larva propels itself along the substrate by ciliary motion, but steering is effected by lateral contractions of the body [36]. A slightly more complex organization is described for the acoel flatworm Convoluta pulchra [37]. Here cilia also drive the worm forward, but muscle allows the worm to change its shape and direction of movement as well as to position its mouth and ingest food.…”