<i>Procephalothrix filiformis</i>, light micrographs of Azan stained transverse sections of the brain.

<p><b>a</b>: Frontal part showing the huge layer of neuronal cell somata (<i>cs</i>). <b>b</b>: Four cephalic cords (<i>cc</i>) extend towards the tip of the head. <b>c</b>: A dorsal commissural tract (<i>dct</i>) connects the two halves of the brain (<i>br</i>). The neuronal cell somata are separated from the neuropil of the brain by an inner neurilemma (<i>in</i>). The brain is embedded in a layer of longitudinal muscles (<i>lm</i>), which lie underneath a layer of circular muscles (<i>cm</i>). <b>d</b>: The brain (<i>br</i>) is divided into dorsal (<i>dl</i>) and ventral (<i>vl</i>) lobes in its posterior part and composed of a central neuropil (<i>np</i>) which is surrounded by cell somata (<i>cs</i>). The somata are separated from the neuropil by an inner neurilemma (<i>in</i>), and two proboscidial nerves (<i>pn</i>) are present. <b>e</b>: A dorsal nerve (<i>dn</i>) arises from the dorsal commissural tract inside the musculature and migrates dorsally, a ventral nerve (<i>vn</i>) from the ventral commissural tract. Neurites (<i>ne</i>) of the dorsal nerve extends as a roof-like structure around the rhynchocoel. <i>mc</i>: medullary cord. <b>f</b>: The lateral medullary cords (<i>mc</i>) are embedded into the longitudinal muscle layer, and the dorsal nerve (<i>dn</i>) is located subepidermally. <i>bv</i> blood vessel, <i>fg</i> foregut, <i>vn</i> ventral nerve.