The Stylet Apparatus of Monostiliferous Hoplonemerteans

Open Access
Publisher Website
Google Scholar
Abstract
This paper compares the ultrastructure, development, and functional morphology of the stylet apparatus in monostiliferous hoplonemerteans (Phylum Nemertea: Class Enopla). The apparatus occurs in the middle region of the worm's eversible proboscis and consists of two main components: 1) a central stylet that is attached to an anchoring device called the basis; and 2) reserve stylet sacs that contain nail-like reserve stylets. The knob-shaped proximal piece of the central stylet is embedded in the anterior end of the basis, and the tapered shaft of the stylet is directed anteriorly. When the proboscis is fully everted, the central stylet is used to stab such prey as polychaete worms and small crustaceans. Wounds inflicted by the stylet allow the introduction of paralytic neurotoxins that are produced by glandular cells in the epithelial lining of the proboscis. Reserve stylets form intracellularly within the reserve stylet sacs and serve to replace the central stylet when it becomes lost or damaged. Fully developed stylets typically measure 50 to 250 μm in length and comprise an organic core surrounded by an inorganic cortex that contains calcium phosphate. In juvenile worms that are four days to several weeks old, reserve stylets are assembled before other components of the stylet apparatus are fully differentiated. Subsequently, a reserve stylet is transferred to the central region of the proboscis and placed on the basis. Stages in the process of stylet attachment are described, and the probable sequence by which the stylet apparatus co-evolved with toxin-producing cells is discussed.

This publication has 0 references indexed in Scilit: