Anatomy and ultrastructure of the proboscis in Mesorhynchus terminostylis (Platyhelminthes, Rhabdocoela)

Abstract

The ultrastructural organization of the proboscis in Mesorhynchus terminostylis is distinctly different from that in other members of the Polycystididae in which it is currently classified. The sheath epithelium is formed by three belts, all with intra-epithelial nuclei. The apical belt of the bipartite cone epithelium has a single intrabulbar nucleus, and the basal belt possesses five insunk nucleiferous cell parts behind the bulb. Six types of glands surface through the epithelia; the three types emerging through the cone epithelium can be homologized with those described for Polycistis naegelii. Only uniciliary receptors are found in the epithelium. The musculature in the bulb has a very loose appearance, and the bulbar septum appears to be a bipartite basement membrane. The septum can be considered the basement membrane of the cone epithelium as if the contractile portion of the inner longitudinal muscles have invaded the epithelium and come to lie between the epithelial cells and the basement membrane. Thus the inner musculature of the bulb is entirely intraepithelial as is the case for Psammorhynchus tub&penis and Cytocystis clitellatus. The systematic position of M. terminostylis remains uncertain but seems to lie between Psammorhynchus and Cytocystis on one hand and Koinocystididae and Polycystididae on the other.

Molecular phylogeny of Kalyptorhynchia (Rhabdocoela, Platyhelminthes) inferred from ribosomal sequence data

“…Mesorhynchus terminostylis was originally described as a somewhat aberrant member of Polycystididae (Karling ), but based on both ultrastructural and light‐microscopical features of the proboscis De Vocht () came to the conclusion that the species should be excluded from Polycystididae. This conclusion is also endorsed by the fact that the four teeth at the proximal pharynx opening are lacking and that the exact position of Me.…”

Molecular examination of kalyptorhynch diversity (Platyhelminthes: Rhabdocoela), including descriptions of five meiofaunal species from the north-eastern Pacific Ocean

“…47 polycystidid genera, according to Cannon, 1986). For example, Mesorhynchus terminostylus was originally placed within Polycystididae, but differences in the proboscis ultrastructure of this species compared with other polycystidids led some authors to doubt this placement (DeVocht, 1991). However, the phylogenetic tree inferred from our data does place M. terminostylus within the Polycystididae.…”

A co-operative effect of closely spaced intruding objects puncturing into a soft solid

“…Incision of so solids by insertion of a set of closely spaced sharp and pointed objects is a common occurrence in several natural and practical settings, e.g. the feeding apparatus of almost all organisms: 1 the teeth of different shapes and sizes in mammals, 2 rodents 3 and sharks, 4,5 proboscis or the stylus for insects like mosquitoes [6][7][8] and man-made devices like the hypodermic syringe needles useful for a host of biomedical applications. Despite dissimilarity in form and function, among many different naturally occurring feeding devices, there is one aspect which is common: they all appear, not as a single, individual unit, but as a composite device consisting of several closely spaced constituent sub-units.…”

“…Similarly, the maxilla of mosquito proboscis comprises of not one but two puncturing tools both of which are used for incising the host tissue. [6][7][8] Although, their structural details in the context of "feeding" have been worked out to a sufficient extent, how exactly the mechanical processing e.g. cutting and piercing of host tissue, tearing and chewing of food and so on get affected by the close proximity of these tools is not known.…”