In the evolution of metazoans, the stage of development after that of the simple three-layered structure of the flatworms seems to be the appearance of a coelom, or fluid-filled cavity between the body wall and the gut. Another feature that appears at this stage is metamerism, or segmentation, displayed particularly in the annelids. Ail of these animals share some features but are too different to be placed under one phylum, so each has its own.
This group, known as nemerteans, do not have a true coelom. Instead, a solid tissue called a parenchyma fills the cavity between the gut and the body wall. In this respect nemerteans resemble the flatworms. But nemerteans are more highly specialized, with more complex nervous and circulatory systems, and an alimentary canal with a mouth and anus.
Most of the 900 or so species of nemerteans are marine borrowers and feed on invertebrates. They have a remarkable food-catching tube called the proboscis, which is shot out by water pressure through a pore in the head. In many species the proboscis simply coils around the prey but in some it has teeth or a spine that injects poison into its prey.
This group, the nematodes, found in most environments, and millions may occur in only a couple of acres of soil. Scientists have discovered between 10,000 and 20,000 species of nemotodes, and some estimate that tens of thousands more remain undiscovered.
Nematodes have cylindrical, tapered bodies that are covered in a thick protein layer, called the cuticle, which is shed periodically as the animal grows. Water flows in and out of the cuticle and body wall continuously.
The intestines of these organisms run from the mouth to the anus and are enclosed by longitudinal muscles along the length of the body. Roundworms move like snakes by contracting these muscles, helped by the elasticity of the cuticle and the pressure of the fluid in the pseudocoel. They have a simple nervous system, as do most aschelminths. The brain is located at the front end of the body and is made up of a ring of ganglia (bunches of nerve tissue), from which nerves run down the length of the body.
Nematodes have separate sexes and give birth to larvae that resemble the adult. Many are parasitic—some only in the larval stage, others only when they are adult, and still others are parasitic throughout their life. Some parasitic nematodes damage crops by sucking the contents from punctured plant cells or by feeding on the tissues inside the plant.
Nematodes also parasitize animals, including humans. The hookworm does great harm by feeding on the blood and cells of the intestinal lining; female hookworms produce many eggs, which leave the host’s body in the feces. In unsanitary conditions, the eggs hatch, and the young hookworms enter the human body by boring through the skin of the feet.
Annelids, members of the phylum Annelida, are worms whose body is divided into many segments. As such, they are a little more advanced than the unsegmented nemerteans and nemotodes. The phylum Annelida contains about 12,000 species divided into three classes: Polychaeta, Oligochaeta, and Hirudinea. In the polychaete ragworm each segment of the body—apart from the head and the last segment—is identical, and the external and internal organs are repeated in each segment. In the oligochaetes, such as the earth worms, and the Hirundinea, or leeches, the segments are not all identical and some are specialized for particular functions. Annelids have a true coelom.
About 70 per cent of all annelids belong to the class Polychaeta. Almost all polychaete annelids are marine. Carnivorous polychaetes typically have a well-developed head region with eyes, sensory tentacles, and powerful jaws. They move by using the fleshy, leglike extensions of the body, called parapodia, on each segment. Each parapodium has many stout bristles called setae. Some polychaetes, such as the ragworm, are fast-moving predators. Others, such as lobworms, burrow in mud, while still others, such as the fan worms, live in tubes.
Most of the 3,000 or more species of oligochaetes are burrowers and live in freshwater or land habitats. Oligochaetes lack, and they have fewer setae than polychaete. Oligochaetes worms travel by peristaltic contractions, as do burrowing polychaetes, using the bristles as a temporary anchor. The familiar earthworm is an oligochaete.
Polychaetes and oligochaetes reproduce sexually. However, polychaetes occur in separate male and female form, while oligochaetes are hermaphroditic, each worm having both male and female sex organs. The sex organs are located toward the front end of the animal in different segments. During copulation earthworms come together in opposite directions with their undersides pressed together. They are held together by a tube of mucus that is secreted by the glands in the clitellum, an enlarged ringlike segment on the body. The clitellum of each worm attaches itself to the segments containing the spermathecae of the other worm. Sperm is moved to the clitellum by muscular contractions and passes through a groove in the clitellum into the spermathecal opening of the opposite worm. After a few days, the clitellum secretes a hard ring of material, which slips forward and collects eggs and sperm as it moves over the genital openings. When it comes off the worm, the ends seal up. This cocoon can carry 20 eggs, which hatch after about 12 weeks.
Leeches are found in fresh water, the sea, and on land. They share several features with oligochaetes, including hermaphroditism, fertilization in a cocoon secreted by a clitellum, and the lack of parapodia. However, all but one species lacks a coelom. Most leeches are parasitic.
Leeches have flattened, muscular bodies with a sucker at each end, which is used as an anchor while they move by extending and contracting the body. Most leeches feed on the surface of their host. The front sucker is clamped onto the prey’s skin, and the teeth in the leech’s mouth then make a small cut, which is unnoticed by the host due to an anesthetic that is secreted by the leech. While the leech sucks blood out of the wound, it secretes a chemical called hirudin, which prevents the victim’s blood from clotting. Bloodsucking leeches feed infrequently, but when they do feed they can draw out several times their own weight in blood in one meal.