The Nemertines

A short report about the Nemertines, its characteristics, reproduction, and classification.

1. The Nemertines Report By: Veronica Baje

2. What are Nemertines? The Nemertines or Nemerteans are non-parasitic unsegmented worms; most are marine, but there are a few freshwater species, and even a few species that live in moist tropical habitats on land. The body is slender, worm-like, soft, highly contractile, unsegmented, bilaterally symmetrical, triploblastic and acoelomate.

3. General Structure The body is nearly always narrow and elongated, cylindrical or depressed, un-segmented and devoid of appendages. In length they vary from a few millimeters to as much as twenty-seven metres. In some cases, there is a short narrower posterior region or tail. The distinct head is absent. The entire surface is covered with vibratile cilia and frequently the integument is vividly coloured. Gland cells of the epidermis secrete a mucous matter, which may serve as a sheath or tube for the animal.

4. Body Wall of Nemertine The body wall is consists of epidermis, dermis, circular muscle layer and longitudinal muscle layer. The outermost layer of the body wall is an epidermis of columnar cells, many of which are ciliated, while others are unicellular glands, some of which are arranged in groups. The unicellular glands secrete mucus with which the surface is usually covered and may form gelatinous tube.

5. The Proboscis The proboscis of a Nemertine is an elongated, coiled, hollow and muscular organ opening in front and closed behind. It lies within the proboscis sheath and retractile in nature. 

6. Digestive System The nemertine show a very well-developed digestive system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine.

7. Respiratory and Circulatory System Nemertines lack specialized gills, and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid circulation rather than diffusion to move substances through their bodies.

8. Nervous System The central nervous system consists of a brain and paired ventral nerve cords that connect to the brain and run along the length of the body.

9. Excretory System Nemertea use organs called protonephridia to excrete soluble waste products, especially nitrogenous by-products of cellular metabolism.

10. Reproduction Larger species often break up when stimulated, and the fragments often grow into full individuals. Some species fragment routinely and even parts near the tail can grow full bodies. Most reproduce sexually, and most species are gonochoric (the sexes are separate),but all the freshwater forms are hermaphroditic

11. Gonochoristic – (dioecious) Most nemertine species have separate sexes. Protandric – Hermaphrodite nemertine species that are protandric . Fertilization is usually external to most species, the embryo develops into an elongated ciliated, microscopic larval stage.

12. Advanced stages of juvenile development inside pilidium of  Micrura alaskensis

13. Classification of Nemertines Anopla – Members of the Anopla lack stylets; that is, the proboscis is unarmed. The mouth of anoplan nemertines is posterior to the brain. Enopla – The proboscis of the Enopla may be armed with a stylet—with all of the armed species belonging to a single order—and the mouth is anterior to the brain.

14. Summary The nemertini are the simplest eucoelomates . These ribbon-shaped animals bear a specialized proboscis enclosed within a rhynchocoel . The development of a closed circulatory system derived from the coelom is a significant difference seen in this species compared to other pseudocoelomate phyla. Alimentary, nervous, and excretory systems are more developed in the nemertini than in less advanced phyla. Embryonic development of nemertine worms proceeds via a planuliform larval stage.