Chaeta and chaetogenesis in <i>Thalassema thalassemum</i> reconstructed from a stack of semi-thin sections (Direct link: www.morphdbase.de/?E_Tilic_20140929-M-21.1).

<p>A. Chaetal sac with four developing chaetal follicles (<i>CF1</i>–<i>CF4</i>) and one completed follicle (<i>CF5</i>). Developing chaetae (<i>dCH</i>) in <i>CF3</i> and <i>CF4</i> and complete chaeta (<i>Ch</i>) of CF5 are <i>dark grey</i>. Chaetal muscles are not shown. B. Maximum projection of a CLSM stack of the basal section of CF5 with propidium iodide staining of nuclei (<i>cyan</i>) and autofluorescent chaeta (<i>yellow</i>). Note large nucleus (<i>nc</i>) of chaetoblast. <i>bwm</i> body wall muscles, <i>cb</i> chaetoblast, <i>Cu</i> cuticle, <i>ecm</i> extracellular matrix, <i>ep</i> epidermis, <i>ns</i> ventral nerve cord, <i>pe</i> peritoneum.</p

Schematic drawings of the peripheral nervous system in nemerteans.

<p><b>a</b>: Scheme of the different nerve plexus in nemerteans. <b>b</b>: Scheme of the different major nerves present in nemerteans. Note the location of the neuronal cell somata (<i>blue</i>) of the medullary cords. The neuronal cell somata overlie the neuropil of the medullary cords dorsally and ventrally. <i>pr</i> proboscis, <i>ry</i> rhynchocoel.</p

<i>Callinera grandis</i>, light micrographs of Azan stained transverse sections of the brain.

<p><b>a</b>: The cephalic nerves (<i>cn</i>) are composed of neuropil (<i>np</i>) and neuronal cell somata (<i>cs</i>), a cephalic gland (<i>cg</i>) is located dorsally. <b>b</b>: The anterior region of the brain (<i>br</i>) is covered by a enormous layer of cell somata (<i>cs</i>). <b>c</b>: The brain is composed of a central neuropil (<i>np</i>) and a surrounding layer of cell somata (<i>cs</i>). The two halves of the brain are connected by ventral (<i>vct</i>) and dorsal (<i>dct</i>) commissural tracts. <b>d</b>: Posteriorly the brain is divided into a ventral (<i>vl</i>) and dorsal (<i>dl</i>) section. A dorsal nerve (<i>dn</i>) arises from the dorsal commissural tract, and the paired esophageal nerves (<i>en</i>) arise from the ventral commissural tract. Shortly anterior to the foregut a concentration of neurons (<i>gn</i>) occurs which are associated with the esophageal nerves. The somata are separated from the neuropil by an inner neurilemma (<i>in</i>), and the whole brain is enclosed by an outer neurilemma (<i>on</i>). <b>e</b>: The ventral lobes of the brain are confluent with the lateral medullary cords (<i>mc</i>). <i>mo</i>: mouth opening, <i>pr</i>: proboscis. <b>f</b>: The lateral medullary cords (<i>mc</i>) run to the posterior of the animal. The cell somata (<i>cs</i>) cover the neuropil in a C-shaped manner. <i>fg</i> foregut.</p

Different hypotheses on the evolutionary transformation of annelid and capitellid segmental chaetae (A) into echiuran ventral (B) and caudal anal chaetae (C).

<p>Annelid chaetae are formed on the ventral edge of the notopodial chaetal sac and on the dorsal edge of the neuropodial chaetal sac. Developing chaetae are <i>red</i>, completed chaetae are <i>black</i>, chaetal sac is <i>grey</i>. All structures that are reduced according to the different hypothesis are <i>paler</i>; presumably reduced chaetal sacs are marked by a <i>dotted line</i>. <i>Arrows</i> mark expansion or shifting of chaetal sac. <i>H1</i> hypothesis one: notopodial chaetae have been reduced and the neuropodial chaetal sac shifted ventrally. <i>H2</i> hypothesis two: notopodial chaetae plus one neuropodial group of chaetae have been reduced and the remaining neuropodial sac shifted ventrally. <i>H3</i> hypothesis three: all chaetal sacs expand dorsally and ventrally, respectively. <i>H4</i> hypothesis four: neuropodia are reduced and the notopodial chaetae expand dorsally and ventrally.</p

<i>Tubulanus polymorphus</i>, confocal laserscanning (cLSM) micrographs of differently immunostained vibratome sections.

<p><b>a</b>: Transverse section, anti-FMRF (<i>red</i>), anti-α-tubulin (<i>green</i>) showing the subepidermal (<i>snp</i>) and the stomatogastric (<i>sgp</i>) nerve plexus. <b>b</b>: Horizontal section, anti-FMRF showing the regular arranged neurites (<i>ne</i>) of the subepidermal nerve plexus. <b>c</b>: Horizontal section, anti-FMRF. The neurites (<i>ne</i>) of the intrastomatogastric nerve plexus (<i>isgp</i>) are arranged in a regular ladder-like fashion. d: Transverse section, anti-serotonin. The lateral medullary cords (<i>mc</i>) are interconnected by a commissural plexus (<i>cnp</i>). The commissural plexus surrounds the mouth opening (<i>mo</i>).</p

Schematic drawings of the nemertean central nervous system.

<p><b>a</b>: Dorsal view of the central nervous system, the neuronal cell somata are shown in <i>bright blue</i>. <b>b</b>: Transverse section of the brain. The brain is composed of a central neuropil (<i>gray</i>), which is surrounded by neuronal cell somata (<i>bright blue</i>). The neuronal cell somata may be separated from the neuropil by an inner neurilemma. The whole brain may be enclosed by an outer neurilemma. The two halves of the brain are interconnected by a dorsal (<i>dct</i>) and a ventral commissural tract (<i>vct</i>) which form a ring around the rhynchocoel (<i>ry</i>). <i>dl</i> dorsal lobe of brain, <i>pr</i> proboscis, <i>vl</i> ventral lobe of brain.</p

<i>Callinera grandis</i>, light micrographs of Azan stained neurons.

<p><b>a</b>: Transverse section, overview of the brain; position of the three different types of neurons (<i>S1</i>–<i>S3</i>); all nuclei stain purple. <b>b</b>: Higher magnification of somata of type 1 and 3 neurons (<i>S1</i>, <i>S3</i>). Cell bodies are circular; those of S1 are arranged in clusters, those of <i>S3</i> are most prominent. <b>c</b>: Higher magnifcation of somata of type 2 and 3 neurons. <i>S2</i> cell bodies are pear shaped and slightly enlarged. The neurites (<i>ne</i>) of the brain branch into the cell somata layer. <i>np</i> neuropil.</p

<i>Carinina ochracea</i>, light micrographs of Azan stained neurons.

<p><b>a</b>: Cell bodies of type 1 neurons (<i>S1</i>) are not enlarged while those of type 2 neurons (<i>S2</i>) are; nuclei stain purple. <b>b</b>: Neuronal cell somata of the medullary cord (<i>mc</i>) are of the second type described for the brain. <b>c</b>: The canal of the cerebral organ ends in a layer of special neuronal cell somata (<i>sco</i>), which are not present in the brain.</p

<i>Tubulanus polymorphus</i>, light micrographs of Azan stained neurons.

<p><b>a</b>: Transverse section, overview of the brain; position of the three different types of neurons (<i>S1</i>–<i>S3</i>). <b>b</b>: Higher magnification of the somata of neuron types 1 and 2. Nuclei of type 1 neurons (<i>S1</i>) stain purple; the cell body is not enlarged. Nuclei of <i>S2</i> stain red. Type 2 neurons are solitarily distributed all over the brain. <b>c</b>: Nuclei of type <i>3</i> neurons stain red and the cell body is more prominent compared to <i>S1</i> and <i>S2</i>. <i>np</i> neuropil, <i>on</i> outer neurilemma, <i>vct</i> ventral commissural tract.</p

<i>Carinoma mutabilis</i>, light micrographs of Azan stained transverse sections of the brain.

<p><b>a</b>: The cephalic nerves (<i>cn</i>) are circularly arranged around the inner margins of the head. <b>b</b>: The brain (<i>br</i>) is composed of a central neuropil (<i>np</i>) surrounded by cell somata (<i>cs</i>). The whole brain is encircled by an outer neurilemma (<i>on</i>). The two proboscidial nerves (<i>pn</i>) originate from the ventral commissural tract (<i>vct</i>). <b>c</b>: A dorsal nerve (<i>dn</i>) arises from the dorsal commissural tract, the esophageal nerves (<i>en</i>) originate from the ventral commissural tract, the brain is in its posterior part divided into a dorsal (<i>dl</i>) and ventral (<i>vl</i>) lobe. Dorso-lateral nerves (<i>dln</i>) arise from the dorsal commissural tract and merge with the dorsal lobes. d: The ventral lobes are confluent with the lateral medullary cords (<i>mc</i>), two proboscidial nerves are present (<i>pn</i>). Note the different positions of the dorsal nerve (<i>dn1–dn3</i>). e: Higher magnification of the anterior head region, showing the location of the cephalic nerves (<i>cn</i>). <i>dn</i> dorsal nerve, <i>mo</i> mouth opening.</p