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Station 1A
The Annelids – Phylum Annelida
The annelids, or segmented worms , occupy marine, aquatic and aquatic habitats. Annelids are coelomate protostomes and typically, are elongated ‘worm-like’ animals. By far, the most distinguishing characteristic of annelids is metamerism , the serial repetition of body parts which gives them a segmented appearance.
Metamerism is an important evolutionary landmark due to the significance of increasing specialization of body parts. Each segment, called a metamere , can become specialized to accomplish a specific task. Metamerism is not confined to annelids; arthropods and chordates are also segmented.
Segmentation is seen not only in external morphology; internal segmentation of body organs is also quite evident. The coelomic cavity is divided by septa (sing –septum). This allows the hydrostatic pressures to be confined to individual compartments. Altering these hydrostatic pressures in conjunction with the muscular contractions of individual metameres allows for more coordinated movements. Adaptations like these allow annelids to be efficient swimmers, creepers and burrowers.
Annelids have a complete digestive system that exhibits a great deal of regional specialization and increased muscular support. A closed circulatory system and one or more pumping hearts deliver blood to body tissues. Most exhibit pronounced cephalization and have a central nervous system. Metabolic waste removal is accomplished by metanephridia located in each segment. Gas exchange can occur through the epidermis or using specialized respiratory organs.
Examples of Annelids
The Leech
Leeches are common aquatic and marine ectoparasites. A leech’s segmented body undulates as it swims in search of a suitable host. Once located, the leech uses its anterior oral sucker to firmly attach to the host’s body. The host’s integument is perforated and the leech introduces an anticoagulant called hirudin into the open wound.
Blood and body fluids flow from the host into the leech’s digestive tract. The stomach of a leech forms an expansive crop. As the crop fills with blood, the leech expands like an accordion. Once filled, the leech releases and swims to a suitable location where it will attach to the substrate using its posterior caudal sucker to digest its meal.
Leeches have been used medically for centuries and are still an important pharmaceutical source of the anticoagulant hirudin.
The Annelids and Arthropods
Laboratory
Examine the leech under the microscope and sketch your observation in the space below.
Polychaete worm – The Clamworm, Nereis
The segmented polychaetes (“many bristles”) exhibit a higher degree of cephalization than leeches. The clamworm, Nereis , is a marine polychaete which can be found in burrows in soft mud, sand and debris. Clamworms feed on just about anything from detritus to small animals using their pincher-like jaws to rip bits of flesh. Each body segment has a pair of lateral parapodia (sing – parapodium) covered with many stiff bristles called seta. These parapodia help the clamworm move through the substrate and provide additional surface area for respiration.
Examine the preserved specimens, the parapodium slide and label figure below.
Leech
Clamworm- Label metamere, jaws, parapodium, seta
Station 1B
Oligochaetes – The Earthworm, Lumbricus
Oligochaete (“few bristles”) worms have fewer setae associated with each segment. The common earthworm, Lumbricus , prefers moist soil rich with organic matter on which it feeds. Using muscular contractions, hydrostatic pressures and pairs of ventral seta for traction, earthworms burrow into the soil and perform an important ecological service by aerating the soil. Mucus produced by the glandular epidermis keeps the earthworm moist and allow for gas exchange.
Earthworms are monoecious but must mate for cross-fertilization. Special reproductive segments form the clitellum , a broad region located in the anterior one-third of the body. The clitellum secretes thick mucus which will allow for the exchange of sperm and will ultimately encase the fertilized eggs in a cocoon.
Sperm are produced in the testes and mature in the sac-like seminal vesicles which surround each testis. The sperm are received during copulation by the balloon-like seminal receptacles of another earthworm. Eggs from the ovaries are routed through the oviduct and out of the body. Sperm from the seminal receptacles is released and fertilizes the eggs externally in the thick mucus cocoon.
The mouth is located on the first segment and leads to the muscular pharynx. As food is swallowed, muscular contractions of the digestive tract move the food to the esophagus and stomach. The stomach of an earthworm is divided into two distinct regions; an expandable crop for storage and a muscular gizzard. After the food is masticated by the grinding actions of the gizzard, it is passed to the elongated intestine for digestion and absorption. Waste is eliminated through the anus located on the terminal body segment.
The central nervous system of the earthworm includes a ventral nerve cord and a pair of cerebral ganglia (the brain) located on the anterior, dorsal aspect of the pharynx. The closed circulatory system includes a dorsal blood vessel , a ventral blood vessel , five muscular aortic arches (the hearts) which pump the blood. Metabolic wastes are collected in the pairs of tubular metanephridia located in each body segment and are excreted.
Observe the preserved earthworm specimens as well as the model and use the laminated sheets to label the figures below.
Earthworm external anatomy – label mouth, body segment, seta, clitellum, anus
Earthworm cross-section – Label seta, coelom, dorsal blood vessel, ventral blood vessel, epidermis, muscle, lumen of intestine
Earthworm internal anatomy, dorsal view – Label cerebral ganglia, pharynx, aortic arches, esophagus, crop, gizzard, intestine, mouth, anus, dorsal blood vessel, testis, seminal vesicles, ovary, seminal receptacle, metanephridium, clitellum
Station 2
The Arthropods – Phylum Arthropoda
The arthropods (joint foot) represent the greatest diversity of species in the animal world with well over a million different species. Arthropods can be found occupying every habitat on earth.
Arthropods are segmented ( metamerism ) and arising from these body segments are jointed appendages. Unlike the annelids, whose external metamerism shows little true regional specialization of appendages, the arthropods body segments are clearly fused into discrete functional units called tagma (i.e. head, thorax and abdomen of an insect). With this tremendous regional specialization, various appendages evolved to serve a great variety of functions including sensory reception, equilibrium, locomotion feeding and reproduction.
Arthropods are covered by a hardened exoskeleton made of chitin. This non-cellular exoskeleton is secreted by the epidermis and must be periodically shed by molting ( ecdysis ) as the arthropod grows. The exoskeleton not only provides mechanical protection and support but provides a rigid attachment for powerful muscles.
Arthropods are coelomate protostomes and have pronounced cephalization, a well-developed central nervous system, complete digestive system and an open circulatory system. Respiratory structures vary by group and include tracheal tubes, gills and book lungs. Most species are dioecious, some are sexually dimorphic while others have complex social caste systems.
Arthropods may be classified into several lineages based upon the tagmata present, structure of the appendages (as biramous with a fork at the distal end or uniramous with a single claw at the distal end) and the number of antennae present.
Examples of Arthropods
Generalized trilobite
The trilobites are thought to have been the first arthropods. Trilobites were all marine organisms and are well represented in the fossil record. All trilobites were extinct by the end of the Permian some 250 million years ago.
Observe the trilobite fossils and compare to the illustration.
The Chelicerates – horseshoe crabs, spiders, ticks and scorpions
Chelicerate arthropods have a body which is divided into two tagmata; the cephalothorax and abdomen. The first pair of appendages is the chelicerae , which are feeding appendages adapted for seizing and tearing food. The second pair of appendages is the pedipalps which are also used for feeding. Four pairs of walking legs are used for locomotion. Chelicerates have no antennae.
Horseshoe crab – Dorsal view, right; ventral, left. Label chelicerae, pedipalps, walking legs, telson, compound eye, cephalothorax, abdomen, book gills
Horseshoe crabs are marine bottom-dwellers found along the Atlantic coastline of North America. They have a large protective dorsal carapace which covers the cephalothorax. A pair of compound eyes allows for keen vision. The pincer-like chelicerae are used to catch and manipulate food. The flap-like book gills are ventrally located and are for gas exchange. The flapping motions of the book gills are also used as the horseshoe crab swims. A hard, stiff telson or tail spine extends from the abdomen. This telson is used to anchor the animal when burrowing or to right itself if turned over.
Scorpions are predatory chelicerates that are common in arid to semi-arid habitats. The chelicerae are modified to process food and their pedipalps have evolved into large pinchers used to catch prey. Scorpions also have a modified telson, a stinger , used to deliver a venomous toxin to subdue and begin digestion of their prey. This stinger may also be used in self-defense to prevent unwanted advances from predators looking to make a meal of the scorpion.
The crustaceans – crabs, shrimp, crayfish and barnacles
The crustaceans are gill-breathing arthropods with biramous appendages. The body is divided into the cephalothorax and the abdomen. Mandibles are used to masticate food and several pair of maxillae located on the head are used in food handling. A large sheet of the exoskeleton known as the carapace covers and protects the cephalothorax. Crustaceans use two pairs of antennae for chemoreception, tactile sense and equilibrium. Most crustaceans are aquatic (i.e. crayfish) or marine (i.e. lobster and blue crabs) although there are a few terrestrial examples (i.e. sow bugs or pill bugs).
Scorpion, dorsal view
Crustaceans – goose barnacle, left; crayfish, right
Review Questions
- Name the earliest known arthropods.
- What does the term ‘tagma’ mean?
- Describe the difference between a biramous and a uniramous appendage.
- Why must an arthropod undergo ecdysis?
- Describe the use of a telson by a horseshoe crab.
- Name the structures a horseshoe crab uses for respiration.
- Spiders, horseshoe crabs and ticks belong to what sub-group of arthropods?
- Name the appendages used by a spider to spin its silken web.
- Name the vector of Lyme’s disease.
Hexapods – The Insects
Insects are by far the most diverse group of animals, much of the diversity seen in arthropods is due to insect diversity. Well over a million species of insects have been described and new species are discovered all the time. Insects occupy many terrestrial and aquatic habitats. Insects have also had profound effects on human society as many forms may be vectors of deadly diseases, are annoying pests or beneficial pollinators of plants.
The body of an insect is divided into three tagmata; the head, thorax and the abdomen. The appendages which arise from these tagma are uniramous. A single pair of sensory antennae arises from the head and compound eyes allow for vision. The appendages around the mouth are extremely specialized in the various groups of insects. Some, as in the butterfly, have mouth appendages modified into hollow tubes for feeding on nectar deep in flower whorls. Others, like grasshoppers and beetles have chewing mouth parts modified for tearing plant material or flesh.
Three pairs of walking legs (hexapod = ‘six foot’) and two pairs of wings arise from the thorax. Again, a tremendous degree of specialization in these appendages is seen in the insect groups. Grasshoppers have large hind legs for jumping and powerful wings for short, quick flights. The ectoparasitic lice have lost their wings through the evolutionary process and their walking legs are adapted to clinging to the hair or feathers of their host. Spiracle openings are also located on the thorax and may be found laterally on the abdominal segments as well. The abdominal segments may house the genitalia, external appendages used during copulation. Insects are dioecious and most are sexually dimorphic.
Insect life cycles follow two basic patterns: incomplete metamorphism and complete metamorphisms (metamorphic = many forms). Incomplete metamorphism is seen in insects such as grasshoppers where the egg hatches into larvae which resembles the parent but is much smaller, may lack fully developed wings and is sexually immature. Gradually, the larvae grows (and molts) and increases size until adulthood. In complete metamorphism , such as in a butterfly, the change is much more dramatic between the stages. The egg hatches into larvae, which oftentimes does not resemble the adults at all. After several larval stages, a pupal stage occurs where the final changes are made in a cocoon or case.
In some insects, there is a well-defined social structure. Bees, ants, and termites are all known to form large colonies which consist of reproductive individuals (queens and drones) and workers. The workers do the majority of the construction of the hive or mound as well as the collection of food and provide protection.
Insect Diversity - Common Insects (Illustrations not to scale)
Grasshopper – Label head, thorax, abdomen, antennae, spiracles, wings, walking legs, compound eye
Gulf Coast Fritillary life cycle – label larva (caterpillar), pupa in cocoon, feeding adult
June beetle Crab louse
Fruit fly
Worker ant
Class Activity
Crayfish Anatomy and Specialization of Appendages
You will need a dissection pan, two dissection probes (needles), a pair of forceps and scissors to complete the exercise.
Obtain a two crayfish (male and female) from your instructor and place in the dissection pan.
Using the laminated sheets and information below, identify the external structures of your crayfish and label the illustrations.
The crayfish’s body is divided into the cephalothorax and the abdomen. Like most crustaceans, the dorsal aspect of the cephalothorax is covered by a hardened sheet of exoskeleton called the carapace. You may notice the transverse cervical groove which marks the location where the head and thorax are fused together. If you carefully lift the posterior end of the carapace, you can see the feather-like gills which are attached to the larger appendages.
The anterior portion of the carapace forms the pointed rostrum which covers and protects the eyes and the base of the antennae. There are two pairs of antennae; a smaller forked pair of antennules and a larger pair of antennae. Both provide sensory information such as taste, smell and equilibrium.
Surrounding the mouth are a pair of serrated mandibles used for mastication of food. Posterior to the mandibles are two pairs of maxillae , small flat appendages used in food handling and water circulation under the carapace and across the gills. Posterior to the maxillae are three pairs of maxillipeds which assist with food handling and taste.
The crayfish has five pairs of walking legs. The first pair, the chelipeds , are enlarged to form pinchers used in prey capture and defense. The remaining four pairs of walking legs are used in locomotion. The three pair of maxillipeds and the five pair of walking legs each have a gill associated with their bases under the carapace.
The abdominal segments have appendages in the ventral sides. Each segment has a pair of swimmerets which are used in locomotion and reproduction. In male crayfish, the first two pairs (anterior) are enlarged and are known as copulatory swimmerets. They are used to transfer sperm to the seminal receptacle of the female crayfish during copulation. After fertilization and egg production, the female uses her swimmerets to carry the eggs and young crayfish once they emerge.
The anus opens on the ventral side of the telson and is flanked laterally by a pair of uropods. The uropods and telson form a fan which is used to produce rapid backwards locomotion as the powerful abdominal segments contract.
Crayfish Anatomy
Crayfish – Label mouth, anus, antennae, antennules, eye, rostrum, carapace, cervical groove, cephalothorax, abdomen, mandible, maxillae (1st^ and 2nd^ ), maxillipeds (1st, 2 nd, 3 rd), cheliped, walking leg (2 nd-5 th), swimmerets, copulatory swimmerets, uropod, telson, seminal receptacles
Dorsal view
Ventral view- female
Close-up of mouth region
Ventral view - male
