Anterior Portion
The prostomium, the clitellum and segments between them constitute the anterior portion of the worm, ca. 1/3 of the full length, where the internal organs are relatively gathered.
Nervous System: There is the brain, long eggplant-shaped in lateral view and inverted pear-shaped in dorsal or ventral view, 96 ± 14 µm long (n = 40), 19 ± 3 µm wide at the neck (n = 40), 48 ± 7 µm wide (n = 20) at the widest part, and 38 ± 6 µm thick (n = 20), located in I, II and partial III (Figs. 3A-C). The brain bifurcates in front part (Fig. 3B), and is rounded in posterior margin (Fig. 3C). It extends several cranial nerves, two thicker ones forward to the sensory papillae on the epidermis of the prostomium, two backward to the pharyngeal pad, and also stretches circumpharyngeal connectives downward to the subpharyngeal ganglia in II, linking the ventral nerve cord, 26 ± 9 µm wide and 18 ± 3 µm thick (n = 15), till the last segment before the pygidium (Fig. 10). The ventral nerve cord exposes only a little in I (Figs. 3D, E), and is relatively wider and thicker in II, III and IV than that in the rest segments (Figs. 3A; 4D-G; 5D, G).
Digestive System: The mouth opens on the ventral side of the peristomium (Figs. 3A, D), which is followed by the buccal cavity, pharyngeal pad, esophagus, intestine and anus (Figs. 3D; 4–10). The dome-like pharyngeal pad, 92 ± 16 µm long (n = 32), 77 ± 11 µm wide (n = 11) and 40 ± 6 µm thick (n = 21), is rich in muscles and has, at the top, a muscular knot and ring (Figs. 4A-E, G). The knot extends many retractor muscles to attach to the body wall on dorsal, lateral and ventral sides (Figs. 4A-C, E, G), and the ring encircles the anterior part of the peptonephridia (esophageal appendages) and esophagus (Figs. 4C, F, G). The peptonephridia, 48.6 µm long and 22.2 µm in diameter at its anterior part (Figs. 4A-C), is attached to the back of the pharyngeal pad; the rest have two branches, coiled and hyaline, 96.4 µm long; the full length is 145 µm and the whole is confined in IV (Figs. 4A-C, F, G). Each segment from IV to VI has a pair of pharyngeal (septal) glands, surrounding esophagus and squeezing the postseptum of the segment to the rear, especially in IV and V (Figs. 5A, B). The pharyngeal glands in IV and V are composed of elliptic dorsal and ventral lobes and thin strands, and the dorsal lobes are larger than the ventral ones (Figs. 5A, B; Table 1). The pharyngeal glands in VI are also made up of dorsal and ventral lobes and thin strands, but sometimes there appears a smaller middle lobe between them (Fig. 5B). The dorsal lobes in IV and V are separate (Fig. 5B), but the ones in VI connected dorsally (Figs. 5B, I). Their sizes are listed in Table 1.
Table 1
Sizes of the elliptic lobes of the pharyngeal glands in IV, V and VI, in micron (µm).
Pharyngeal gland in | IV / length × width | V / length × width | VI / length × width |
Dorsal lobe (m ± s) | (87 ± 17) × (60 ± 13) | (79 ± 13) × (60 ± 14) | (72 ± 18) × (39 ± 13) |
Ventral lobe (m ± s) | (64 ± 21) × (39 ± 9) | (86 ± 17) × (34 ± 10) | (92 ± 22) × (39 ± 7) |
Sample size (n) | 45 for DL, 24 for VL | 50 for DL, 21 for VL | 33 for DL, 35 for VL |
m, mean; s, standard deviation; DL, dorsal lobe; VL, ventral lobe |
Circulatory System: The dorsal vessel, 6.8 ± 0.5 µm in diameter (n = 5) in contraction, originating from XI, runs along the dorsal midline of the intestine (Fig. 6A) and esophagus (Figs. 5B; 4B), forward passes through retractor muscles above the pharyngeal pad (Figs. 4D-G; 3A), extends into the sinus beneath the brain (Fig. 3A), and then forks laterally and turns downobackward into lateral vessels (Figs. 4E-G), which merge underneath esophagus between ventral lobes of pharyngeal glands in IV (Figs. 5C, D) into the ventral vessel (Figs. 5G; 6A; 7C, E), 10.5 ± 3.4 µm in diameter (n = 5), and going backward till last segment of the body (Figs. 8A, D-F; 9B-F). Propelled by cyclic contractions of muscles along the intestinal and body walls, the blood flows forward through dorsal vessel in vivo, causing the vessel and each segment to expand and shrink regularly (Figs. 4D-G; 5D, G). Besides, there rises a series of intestinal parietal vessels in all the segments between the clitellum and the pygidium (Figs. 8A, F; 9B-F), which absorb nutritional liquids from intestinal parietal cells and oxygen dissolved in coelomic fluids, and turn them into fresh colorless blood. The blood moves forward in the intestinal parietal vessels with the rhythmic waves of contraction (Fig. 9D) and relaxation (Figs. 9E, F) of circular and longitudinal muscles attaching to the body wall in vivo. When reaching the clitellar segments, the blood pushes the intestine and mature eggs aside, passes through, enters the inlet of dorsal vessel in XI (Fig. 7B), flows onwards to the sinus underneath the brain (Figs. 3A, D, E), and then returns from lateral and ventral vessels in vivo.
Coelomocytes are seen to take place in the coelomic cavity and appear in two forms, nucleate and anucleate. A mass of nucleate coelomocytes, spindle-like, usually 12.9 µm long and 3.2 µm wide, are wrapped in a slender bag, which is hung under dorsal wall of some segments. Anucleate coelomocytes are oval or elliptical, 16.4 ± 2.4 µm long and 8.8 ± 2.1 µm wide (n = 63), floating in the coelomic cavity (Figs. 3D, E; 4D-G; 5D, G; 9D, F).
Excretory System: The nephridia are spindle-shaped, 126 ± 28 µm long and 31 ± 5 µm wide (n = 16) excluding tubules; there is a pair of them in each of the segments from VII to X, in septa 6/7, 7/8, 8/9 and 9/10, lying on both sides of the ventral nerve cord (Figs. 5H; 6A, B). In addition, there are similar structures in each of the segments between the clitellum and the pygidium (Figs. 9B, C, E; 10A).
Reproductive System: Male genital organs: There is a pair of elliptic testes, 14.0 ± 0.9 µm long and 9.7 ± 0.0 µm wide (n = 5), lying both sides of the ventral vessel and attaching to the anteseptum of XI closely (Fig. 7C). Nearby is a pair of seminal vesicles in different shape and size, 122 ± 34 µm long (n = 81), 102 ± 34 µm wide (n = 36) in lateral view and 74 ± 25 µm wide (n = 45) in dorsal or ventral view, with many spots on their surfaces (Figs. 7A, C, E). Slightly backward is a pair of sperm funnels, composed of a collar and an ampulla, vase-shaped without base, 105 ± 26 µm long (n = 36), 57 ± 21 µm wide (n = 24) in lateral view and 69 ± 15 µm wide (n = 12) in dorsal or ventral view (Figs. 7A, C-E; 8E), followed by sperm duct, 3.2–3.5 µm in diameter (n = 3), leading to a pair of penial bulbs on the ventral side of XII. More or less on the collar of the sperm funnels are cilia-like spermatozoa, which drift within XI sometimes. Penial bulbs are compact and elliptic, 81 ± 9 µm wide and 79 ± 9 µm high (n = 26), with thinner ventral epidermis (Figs. 8B-D). XII and XIII are covered by a single layer of clitellar epithelium containing lots of granular unicellular glands, 16.1–19.4 µm in diameter (Figs. 8A-J). Clitellum is saddle-shaped near the ventral nerve cord (Figs. 8D, E).
Female genital organs: Attaching to both sides of the ventral nerve cord and near anterior margin of XII, there appears a pair of grape-like ovaries in which the immature germ cells, oogonia, are located (Fig. 8A); on the same site the figure also shows that XII is filled with many immature eggs that are globular, oval, or in other shape and size, with only a few cut open but the majority covered with many vitelline glandular cells stained blue except their nucleus stained slightly red.
Immature eggs are aggregated into clusters in different shape, and the sizes of the clusters are: 181 ± 55 µm long and 172 ± 56 µm wide (n = 28) in lateral view; 149 ± 45 µm long and 93 ± 24 µm wide (n = 46) in dorsal or ventral view. They are also found in XI (Figs. 7A, C, D), and spirally arranged around two centers in dorsal or ventral view (Fig. 7C). The distance is 90 ± 14 µm (n = 5) between the two centers.
Most spaces within the clitellar segments are occupied by mature (ripe) eggs (Figs. 8C, F-J). Figure 8F shows an oocyte, ootid or ovum is contained in each of the two mature eggs, and Figs. 8G-J illustrate lots of yolk granules present under the stained-blue vitelline glandular cells seen when the cells are peeled off one layer after another. Mature eggs are large and yolk-filled ones, 159 ± 11 µm long and 115 ± 14 µm wide (n = 26), inside the clitellum (Figs. 8C, F-J). When there are more mature eggs, they may extend backward to other segments after the clitellum in vivo.
The unicellular glands in the epidermis of the clitellum secrete cocoon wall, which wraps mature eggs inside and forms cocoons (Figs. 11A-C). Cocoons are elliptic, 503 ± 6 µm long and 340 ± 6 µm wide (n = 3) if containing ca. 8 mature eggs. Mature eggs become bigger after being laid, 228 ± 17 µm long and 170 ± 15 µm wide (n = 12) (Fig. 11C).
There is a pair of spermathecae connecting with their short ectal and long ental ducts, located on both sides of the esophagus and above the ventral lobes of pharyngeal glands in V (Figs. 5A, B, E-G), which open on the ventral body wall, with glandular tissues surrounding their ectal orifice inside (Fig. 5A) and muscular epidermis (cushion) outside (Fig. 5E). The two pieces of muscular epidermis are located along the two longitudinal lines of ventral chaetal bundles, and close to the anteseptum of V (Figs. 4A; 5E). Their ental ducts stretch diagonally forward to and attach to lateral body wall (Fig. 5B). The ampullae of spermathecae are elliptical or spherical, 42 ± 8 µm long and 32 ± 7 µm wide (n = 26), full of cilia-like spermatozoa inside (Figs. 5F, G).
Other Systems: The body wall consists of thin cuticle, epidermal and muscular layers (circular and longitudinal) (Figs. 3A; 5F; 9D, E). Both the muscular layers and the coelomic fluids, through the former acting against the latter, form a hydrostatic skeleton, which maintains the shape and toughness of the organism under living conditions. Exchange between oxygen and carbon dioxide is performed through the moist body wall.