The digestive tract of actinotroch larvae (Lophotrochozoa, Phoronida): anatomy, ultrastructure, innervations, and some observations of metamorphosis

The digestive tract of actinotroch consists of the vestibulum, oesophagus, stomach with stomach diverticulum, midgut, and proctodaeum. Monociliate muscle cells resting on the basal lamina of the oesophagus form its circular musculature. The epithelium of the cardiac sphincter contains axonal tracts and neurosecretory cells. Glandular, secretory, and digestive cells form the epithelium of the stomach and stomach diverticulum. The epithelium of the midgut is biciliate. The proctodaeum is divided into two parts, differing in fine structure and function. Individual serotonian and FMRFamide neurons and fibers occur in the oesophagus, cardiac sphincter, and midgut, as well as surrounding the anus. In larvae of Phoronopsis harmeri Pixell, 1912 during metamorphosis, the larval oesophagus gives rise to the juvenile oesohagus, the upper portion of the stomach stretches and transforms into prestomach, the stomach diverticulum moves into the stomach and then is digested, the larval stomach becomes the juvenile stomach, the midgut gives rise to the pyloric region, and the proctodaeum transforms into the ascending branch of the juvenile digestive tract. The data do not support the views that the proximal part of adult digestive tract forms from the ectodermal epithelium of the dorsal and ventral epidermis of the larva or that the telotroch enters the intestine during metamorphosis.

The fine structure and function of the cephalic appendages of the branchiuran parasite, Argulus japonicus Thiele

The fish parasite Argulus japonicus Thiele (Crustacea: Branchiura) has recently been introduced into Britain and is now established in the wild. A. japonicus , an ectoparasite attaching to, and feeding on, the skin of its host, is a potentially serious pathogen of native freshwater fishes. The anatomy of the attachment and feeding structures is described using light and electron microscopy. The primary attachment organs are the suckers derived from the maxillules. The extrinsic musculature of the suckers comprises two major muscle groups: the suction muscles which insert on the floor of the sucker and generate suction, and the cup muscles which control the orientation and movement of the sucker as a whole. The inner wall of the sucker cup comprises two hoops of thickened cuticle and provides the rigidity necessary to prevent the sucker from collapsing. These hoops are hinged to allow extra movement of the distal hoop plus its marginal m em brane when forming a seal onto the surface of the host. Numerous mucous glands are present in the floor of the sucker. The elongate mouth tube represents a ventral outgrowth of the head bearing a small labrum and labium distally. The homology of the labium is confirmed by the arrangement of its paired muscles, which originate on the undersurface of the ventral cephalic tendon and pass down through channels in the suboesophageal ganglion. The labrum lacks muscles. The ontogeny of the mouth tube, the adult m andibular musculature and the possible feeding mechanism are described. The preoral spine lies in the ventral midline of the body anterior to the mouth tube. It consists of a tapering spine carried on a long eversible sheath. When fully retracted the spine and sheath virtually disappear into the body. As the spine retracts the cuticle of the proximal sheath becomes inverted. The epithelium beneath this sheath cuticle is syncytial and is separated from the cuticle-lined, central duct of the spine by a fluid matrix which can be displaced as the spine is retracted. The fluid matrix appears to be secreted by the epithelial cells of the sheath cuticle. Retraction is by means of paired retractor muscles which originate dorsal to the midgut and pass down through the nerve ring. These muscles shorten to about 25% of their maximum length during extreme retraction.