Internal Anatomy
The cephalic region of Thuridilla contained muscular pharynx, esophageal pouch, central nervous system (CNS), and the unarmed penis (Jensen 1992; Thompson & Jaklin 1988). As shown in figure 1, many structures is compacted within a very narrow area in the anterior part of the body. The parapodia of Elysiidae contain ramifying tributaries of the digestive glands, ovotesties, and albumen gland (Thompson & Jaklin 1988). The reproductive ductile system is very delicate and often associated with robust muscle fibers. The anus of family Elysiidae is located at the top of ciliated groove on the right (Jensen 1992; EOL 2011; Gosliner 1995). The species have short and round pericardium with only one to three pairs of dorsal vessels. Function of this dorsal vessel is still unknown, however, Jensen (1992) suggested it is possible that the structure is association with excretion and osmoregulation. T. bayeri posses a small pharynx, however, the posteroventral pharyngeal pounch with small basal lumen is presence (Jensen 1992). The thin and flat ascus-muscle is located between the ascending and descending limbs of the radula. The ascus is entired internal in the species. The radula teeth is triangular in shape with lateral denticles often short (50-65μm) (Carlson & Hoff 1978; Jensen 1992). Their reproductive system is triaulic and have a distinct pigmented ampulla and larger vagina. T. bayeri have small, flat and elongated stomach with two branches to the digestive gland, posteriorly. General anatomy of T. bayeri is shown in figure 1.
Figure 1: Schematic drawing of partly dissected Thuridilla bayeri. Section of dorsal epidermis has been removed to view the position of internal structure.
Pharynx and Pharyngeal pounch
Figure 2 indicate that there four group of musculature in the sacoglossan pharynx including, dorsal septate muscle, the odontophore, the ascus-muscle, and pharyngeal pouch. Within the odontophore, there are three groups of muscle performing different function: 1) odontophore depressor muscles, 2) radula suspending muscle, and 3) transverse muscle. Species including Thuridilla bayeri, which have pharyngral pouch, often have weakly developed ascus-muscle. Moreover, the separation in between the ascending and descending limbs of the radula is also weak. In T. bayeri. the ascending limb is associated with radula suspending muscles and, where as, the most important structure for piercing the algal cell wall is pharyngeal pouch muscle. Together with pharyngeal pouch, the species also have triangular teeth with lateral denticles allowing them to feed on more rigid algae and calcareous (Jenson 1992). Pharyngeal pouch of the species is a collar shape with small, basal lumen, which is considered plesiomorphic.
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| Figure 2: Schematic drawing of pharynx with pharyngeal pouch modified from Jensen 1992. Abbreviations: ascending limb of radula (a), ascus-muscle (am), ascus (as), descending limb of radula (d), odontophore muscles (om), pharyngeal pouch (pp), lumean of pharyngeal pouch (ppl), radula suspending muscles (rsm), dorsal septate muscle (sm). |
Figure 3: Camera lucida drawing of pharynx of T. bayeri modified from Jensen (1992). Dorsal view (first), lateral view (second), ventral view (third).Abbreviations: longtidutial ascus-muscle (am), buccal ganglion (bg), pharyngeal pouch (pp), dorsal septate muscle (sam).
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Ascus-muscle
Ascus-muscle in T.bayeri is very flat, though anteriorly rather thick and entirely internal. The length of ascus-muscle in Thuridilla seldomly exceed half of the pharynx, this is because the presence of pharyngeal pouch. T. bayeri have ascus located between the ascending and descending radula limbs, inside the odontophore. The ascus is usually rolled up in a spiral manner known as plesiomorphic state (Jensen 1992).
Radula teeth
The evolution of radula in sacoglossa is highly related to the shifts in diet (Jensen 1992; Cimino 1998). T. bayeri show basic radular teeth shape with triangular cusps and lateral denticles (figure 4). Tooth size in the species increase with age in juvenile, but tooth size depend more on diet selection when adult.
Figure 4: Schematic drawing of radular teeth of T. bayeri modified from Jensen 1992
Stomach and Digestive gland:
Stomach of T.bayeri is small and only surrounded with thin wall, which connected with main ducts of the digestive gland. The intestine located at the anterior end and originated from the dorsal surface of the stomach (Jensen 1992).
Notes: Reproduction system anatomy is written on "Reproduction" page under "Behaviour"
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