Microanatomy and ultrastructure of praesoma in acanthellae of Arhythmorhynchus teres Van Cleave, 1920 (Acanthocephala: Palaeacanthocephala)

Abstract

The structure of the praesoma in the acanthocephalan Arhythmorhynchus teres Van Cleave, 1920 at the middle acanthella I stage has been examined by light and transmission electron microscopy methods. Acanthocephalan specimens were retrieved from naturally infected intermediate hosts, Bulychevia ochotensis (Brandt, 1851) (Amphipoda, Talitridae). In the studied acanthellae, the primordia of the proboscis receptacle, proboscis, circular muscles, proboscis retractor, and cerebral ganglion are clearly differentiated in the praesoma. The tegument contained giant nuclei arranged into annular rows oriented transversely to the longitudinal axis of the body. At the anterior pole of the acanthellae, three apical nuclei were present; they were densely grouped and considerably smaller than the giant nuclei. The proboscis primordium consisted of vertical cytoplasmic strands (hook primordia) that, together with the basal cytoplasm of the anterior pole tegument, formed an outgrowth protruding into the acanthella’s body cavity. Thus, similar to the other members of the class Palaeacanthocephala, the proboscis of acanthocephalan A. teres is initially formed in an inverted state. Two layers of muscles surround the proboscis: the proboscis retractor and a layer of circular musculature that closely surrounds the developing proboscis, forming a muscular sheath. Six nuclei are found in certain areas of the cytoplasm of the circular muscles. The tubular proboscis retractor surrounds the proboscis together with the primordium of circular musculature, extends around the cerebral ganglion, and attaches to the base of the proboscis receptacle. Four nuclei at the apex of the inverted proboscis were located in the cytoplasm of the hook primordia, while another two nuclei were in the cytoplasm of the circular musculature primordium.