Constructional morphology and mode of attachment of the trunk of Corynosoma cetaceum (Acanthocephala : Polymorphidae)

Dead specimens of Corynosoma cetaceum mere used to describe the trunk muscu lature of this species and to infer the use of the trunk as a secondary hol dfast. Inferences were based on trunk muscle arrangement, changes in trunk shape, size and distribution of spines, and geometry of tegument thickness. The foretrunk of C. cetaceum is swollen and forms a spiny disk that is ben t ventrally. The disk is flattened by several groups of muscles not describ ed previously, which seem able to finely adjust the disk surface over the s ubstratum. Disk attachment appears to be accomplished by two dorsal neck re tractor muscles specialized in pulling the anchored proboscis into the fore trunk. This mechanism has been described in other acanthocephalans, becomin g surprisingly efficient when used with a flattened, armed foretrunk. The v entrally spined hindtrunk requires force to move downwards in order to atta ch. A single ventral neck retractor muscle seems specialized in pulling the posterior trunk forward, inducing a downward Force due to the muscle's pre cise points of insertion. This mechanism necessarily generates ventral wrin kling that needs to be eliminated for the spiny surface to be functional. T he trunk ventral muscles are apparently arranged so as to concentrate the " excess" of the tegument into a single fold, optimizing the use of the remai ning surface for attachment. The size and distribution of spines, as well a s the geometry of tegumental thickness, conform to these observations. Morp hological changes, seemingly simple, such as structural bending, may have t riggered a cascade of subtle modifications and new functions during acantho cephalan evolution, reflecting how morphological integration and novelty in teract. J. Morphol. 241:237-249, 1999. (C) 1999 Wiley-Liss, Inc.