The proboscis apparatus of the nemertineLineus ruber

Abstract
The proboscis apparatus of the nemertineLinens ruberis divided into three segments: the anterior, middle and posterior proboscis, and in addition to these, the retractor muscle. The latter connects the caudal end of the posterior proboscis to the posterior dorsal wall of the rhynchocoel. The general arrangement of the constituent layers of the three segments of the proboscis is more or less similar: (1) inner epithelium, (2) basement membrane, (3) nerve plexus, and (4) one or two layers of muscle which are covered exteriorly by (5) basement membrane and (6) the endothelial cells. The endothelial cells are freely exposed to the rhynchocoel fluid. The inner epithelium of the anterior and posterior proboscis consists of only one type of lining cells, whereas the epithelium of the middle proboscis has many different types of cells, among which may be mentioned (1) ‘rhabdite’-forming cells, (2) ‘sensory’ cells, (3) cells with long microvilli, (4) mucussecreting cells, and (5) cells with acidophilic granules. The rhabdites of the rhabdite-forming cells are very characteristic. Two stages of the rhabdites have been seen: newly developed and mature rhabdites. In the former, the central tubular core of the structure is small and the ‘pool’ in which the rhabdite is embedded is large. In the mature rhabdite the reverse is true, i.e. the central tubular core is distended with electron-translucent secretion probably derived from the ‘pool’, since the latter is greatly reduced in size. The rhabdites are discharged in clusters into the lumen of the ‘resting’ proboscis and presumably over ‘prey’ when the proboscis is ejected. The muscles of the proboscis have ‘dual’ innervation. Aminergic and cholinergic nerve fibres, which arise from the dorsal cerebral ganglia, enter the proboscis at its anterior connexion (‘hinge’). In the aminergic nerve terminals two types of ‘synaptic vesicles’ have been resolved: vesicles of moderate density (20 to 50 nm) and dense-core vesicles (50 to 80 nm). Cholinergic terminals show typical vesicles of size 20 to 50 nm. The retractor muscle is apparently trebly innervated. ‘Synaptic contacts’ (mostly aminergic) occur at the junction of the proboscis and the retractor muscle. In addition, the retractor muscle has a probable peptidergic type of innervation. Neural terminals loaded with granules of size 140 nm, and thus comparable with other neurosecretory endings, are seen in the close vicinity of the retractor muscle. This histological evidence is supported by the observation that the muscle contracts vigorously when stimulated with oxytocin at a concentration of 0.01 unit/ml. The fluid relationships between the rhynchocoel and the vascular system, that allow the proboscis to be freely ejected and withdrawn, are discussed.