Aschelminths

The aschelminths are the first assemblage of animals to possess a distinct body cavity, but one that lacks mesenteries that are found in more advanced animals

The pseudocoelom is often fluid filled, serves as a cavity for circulation, aids in digestion, and acts as an internal hydrostatic skeleton that functions in locomotion.

Most aschelminths have a complete tubular digestive tract that extends from an anterior mouth to a posterior anus.

It permits, for the first time, the mechanical breakdown of food, digestion, absorption, and feces formation to proceed sequentially and continually from an anterior to posterior direction an evolutionary advancement over the blind-ending digestive system

They are bilaterally symmetrical, unsegmented, triploblastic, and cylindrical in cross section.

Osmoregulatory system of protonephridia.

Phylum Gastrotricha - Gastrotrichs

500 free living marine and freshwater species that inhabit the space between bottom sediments.

They range form 0.01 to 3 mm in length.

Gastrotichs move over the substrate using cilia on their ventral surface.

Phylum Rotifera - rotifers

Derive their name from the characteristic ciliated organ, the carona, located around lobes on the head of these animals.

The cilia of the corona do not beat in synchrony; instead, each cilium is at a slightly earlier stage in the beat cycle than the next cilium in the sequence, creating a metachromal beat.

.1 to 1 mm in length, and are abundant in most freshwater habitats;

2000 species in three classes

Rotifers are usually solitary free-swimming animals, although some colonial forms are known.

Because they are free living males are commonly absent so parthenogenesis is not uncommon.

Usually feed on small microorganisms and suspended organic material.

Phylum Kinorhyncha - Kinorhnchs

are less than 1 mm long , elongate worms found exclusively in marine environments, where they live in mud and sand.

They simply burrow through the mud and sand with their snout feeding on diatoms, algae, and organic mater.

150 species

distinguishing characteristic of 13 definite units called zonites.

Phylum Nematomorpha - horsehair worms

250 species

Hair like nature is so striking that they were formerly thought to arise spontaneously from the hairs of

a horse's tail in drinking troughs or other stock watering places.

The adults are free living, but the juveniles are all parasitic in arthropods.

No distinct head

Larvae must quickly enter an arthropod

Once mature the worm leaves its host only when the arthropod is near water

Phylum Ancathocephala - ancathocephalans

all endoparasites in the intestinal tract of vertebrates (especially fishes)

Two hosts are required to complete the life cycle.

The juveniles are parasites of crustaceans and insects

The body of the adult is elongate and composed of a short anterior proboscis, a neck region, and a trunk .

The proboscis is covered with recurved spines; hence the name spiny-headed worms

1000 species

No digestive system is present; food is absorbed directly through the tegument from the host.

The eggs pass out of the host with the feces and must be eaten by certain insects

Once in the invertebrate, the larva emerges from the egg - hemocoel - eaten by vertebrate.

Phylum Loricifera

Most recently described animal phylum - 1983

Occur in spaces between marine gravel

has a spiny head called an introvert and can be introverted

Phylum Priapulida

15 species

marine worms found in cold waters.

Live buried in the mud and sand of the sea floor, where they feed on small annelids and other invertebrates.

Body is cylindrical in cross section, and ranges in length from 2 mm to 8 cm

Anterior part of the body is an introvert, which can be drawn into the longer, posterior trunk

Phylum Entoprocta

100 species of sedentary marine filter feeders found in coastal waters.

microscopic in size.

An individual entoproct consists of a muscular stalk bearing a cup shaped calyx with a crown of ciliated tentacles.

Some of the most abundant animals on earth

may be 5 billion in every acre of fertile garden soil

10,000 - 50,000 species

Feed on organic mater to living tissues

range in size from microscopic to several meters.

Characteristics

Classification

A typical nematode body is slender, elongate, cylindrical, and tapered at both ends.

Outer noncellular, collagenous cuticle

The cuticle functions to maintain internal hydrostatic pressure, provide mechanical protection, and aid in resisting digestion by the host in parasitic species.

The cuticle is usually molted four times during maturation.

Beneath the cuticle is the epidermis, or hypodermis which surrounds the pseudocoelom.

Contraction of these muscles results in undulatory waves that pass from the anterior to posterior end of the anima, creating characteristic thrashing movements.

Nematodes cannot crawl.

Some nematodes have lips surrounding the mouth, and some species bear spines or teeth on or near the lips.

Sensory organs include amphids, phasmids, or ocelli.

Amphids are anterior depressions in the cuticle that contain modified cilia nd function in chemoreception.

Phasmids are located near the anus and also function in chemoreception.

paired ocelli are present in aquatic nematodes

Nematodes have a complete digestive system consisting of a mouth which may have teeth, jaws, or stylets; buccal cavity; muscular pharynx; long tubular intestine where digestion and absorption occur; short rectum; and anus.

Hydrostatic pressure in the pseudocoelom is responsible for the passage of food through the alimentary canal.

Nematodes accomplish osmoregulation and excretion of nitrogenous waste products with two unique systems.

The glandular system is found in aquatic species, and consists of ventral gland cells, called renettes, that are located posterior to the pharynx. Each gland absorbs waste material from the pseudocoelom and empties it to the outside through an excretory pore.

Parasitic nematodes have a more advanced system, called the tubular system that develops from the renette system.

In this system, the renettes unite to form a large canal, which opens to the outside via an excretory pore.

The nervous system consists of an anterior nerve ring

Most nematodes are dioecious and dimorphic, with the males being smaller than the females.

The long coiled gonads lie free in the pseudocoelom.

Female has vagina that opens to the outside through a genital pore.

Male has single testis and are commonly armed with a posterior flap of tissue called a bursa.

The bursa aids in the male in the transfer of sperm to the female genital pore during copulation.

the development and hatching of the eggs are influenced by external factors, such a s temperature and moisture.

Hatching produces a larva that has most adult structures.

The larva undergoes four molts, although in some species, the first one or two molts may take place before the eggs hatch.

Nematode parasites of humans have a high reproductive potential, life cycles that ensure transmission from one host to another, an enzyme-resistant cuticle, resistant eggs, and encysted larvae.

Ascaris lumbricoides - intestinal roundworm - 800 million people in world - live in the small intestine

of humans. Large number of eggs are produced and exit with the feces.

Enterobius vermicularis are - human pinworm - located in the lower region of the large intestine. At night, gravid females migrate out of the cecum to the perianal area, where they deposit eggs containing a first-stage larva

Necator americanus - New world hookworm - southern US live in small intestine, where they hold onto the intestinal wall with teeth and feed on blood and tissue fluids. Humans become infected when the filariform larva penetrates the skin, usually on the foot.

Trichinella spiralis - porkworm - live in the mucosa of the small intestine of humans and other carnivores. The young larvae encyst in the skeletal muscles and remain infective for many years. The disease caused by this nematode is called trichinosis. Infected meat must be ingested by another host to continue the life cycle. Humans most often become infected by eating improperly cooked pork products.

Wuchereria spp. - Filarial worms - 250 million humans infected - live in lymphatic system where they

block the vessels. - elephantiasis - mosquito

A filarial worm prevalent in the US is Dirofilaria immitis - and is a parasite of dogs. - Heartworm disease