The phasmids are similar in their structure to amphid sensilla, but smaller. They are located at the lateral sides of the tail and enclose the ciliated dendrites of PHA, PHB and, on the left side, PQR neurons as well as one sheath and two socket cells. The cilia of the PHA and PHB neurons extend into the external medium through the channel created by the socket cells. The ending of posterior process of PQR is wrapped by PHso2L. Phasmid neurons were recently shown to function in modulation of chemorepulsion behavior in worms (Hilliard et al, 2002), whereas PQR is suggested to be a mechanosensor.
At L1 stage, a seam cell, T, performs socket cell function and is attached to the sheath cell and the syncytial hypodermis by adherens junctions. At L2 stage, T cell daughters, PHso1 and PHso2 both function as socket cells. PHso1 wraps around the tip of the neuronal dendrites and is connected to the PHsh and PHso2, but not to the hypodermis, by adherens junctions. PHso2, on the other hand, is connected to the hypodermis and Phso1, but not to the PHsh. Later male and hermaphrodite phasmids differ in their composition. In adult male, Phso2 functions a true socket cell whereas PHso1 protrudes into the sheath and may contain up to 2 basal bodies although it doesn't show any other characteristics of neurons. In adult hermpahrodite, on the other hand, PHso1 is the main socket cell and Phso2 has a thin wrapping around it.
Adherens junctions (red arrows in the figure below) seal the support cells to each other, to the hypodermis or seam, and to the ciliated dendrites, holding the elements in place and excluding any flow between external environment and pseudocoelom.
Phasmid sheath cells extend short processes posteriorly into tail tip which swell to form a protective pocket near the phasmid openings for PHA and PHB cilia.
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