Sipuncula
Sipuncula, also known as peanut worms, are a phylum of marine worms. They are unsegmented, bilaterally symmetrical worms with a distinctive introvert, a retractable proboscis used for feeding. The introvert is often covered with tentacles, which help the worm to capture food. Sipunculans are burrowing animals that typically live in soft sediment environments, such as sand and mud. They are found in all oceans of the world, from the intertidal zone to the deep sea.
Characteristics
Sipunculans are characterized by their lack of segmentation, their unbranched digestive system, and their unique introvert. The introvert is a muscular, retractable proboscis that can be extended to feed. It is usually covered with tentacles, which help the worm to capture food. When not in use, the introvert is retracted into the body cavity. Sipunculans also have a distinctive U-shaped digestive tract, with the mouth and anus located at the anterior end of the body.
Environments
Sipunculans are found in all oceans of the world, from the intertidal zone to the deep sea. They typically live in soft sediment environments, such as sand and mud. Some species are also found in rocky crevices and under rocks. Sipunculans are often abundant in shallow water habitats, but they can also be found in deep-sea environments.
Feeding
Sipunculans are deposit feeders, meaning they eat organic matter that is found in the sediment. They extend their introvert into the sediment and use their tentacles to capture food particles. Sipunculans can also be suspension feeders, meaning they capture food particles that are suspended in the water column. They do this by extending their introvert and tentacles into the water column and filtering food particles from the water.
Reproduction
Sipunculans are dioecious, meaning they have separate sexes. They reproduce sexually, with fertilization occurring externally. The eggs and sperm are released into the water, where they are fertilized and develop into larvae. The larvae are planktonic, meaning they drift in the water column. They eventually settle to the bottom and develop into adults.
Classification
The phylum Sipuncula is divided into two classes:
- Sipunculidea
- Phascolosomatidea
There are over 300 species of sipunculans that have been described. Sipunculans are closely related to annelid worms, but they are not considered to be annelids. They are a distinct group of worms that have evolved independently.
References
Wikipedia: Sipuncula
World Register of Marine Species: Sipuncula
National Center for Biotechnology Information: Sipuncula