LABORATORY 2

NEMATODA, ANNELIDA, & ARTHROPODA

This week we’re looking at representatives of the subgrades Pseudocoelomata and Eucoelomata. There is abundant diversity among these three phyla though all members share bilateral symmetry and development includes organ systems. Nematoda is the first animal phylum to exhibit a complete digestive tract. Some nematodes are free living, many are parasitic, some are microscopic though multicellular.

The roundworm Ascaris, a nematode, is parasitic within the intestine of mammals such as humans, pigs, and horses. The parasite is most often introduced into the host organism when food contaminated with eggs is ingested. As you study Ascaris, keep in mind special challenges associated with parasitic existence. What adaptations are evident in this specialized existence?

PHYLUM NEMATODA (Campbell, Chapter 33, p. 607- )

PREPARED SLIDES:

1. Ascaris lumbricoides c.s. male

Label the body wall, dorsal nerve, epidermis, seminal vesicle, sperm cell, testes, cuticle, intestine, pseudocoel, ventral nerve, and muscle cells. Think about the composition of the body wall: cuticle (noncellular), epidermis (cellular), and muscle cells. The muscle is derived from the mesoderm. From what primary germ layer is the intestine derived ? Can you detect muscle tissue adjacent to the endoderm ? (Atlas p. 112)

2. Ascaris lumbricoides c.s. female

Label body wall, dorsal nerve, ventral nerve, uterus, ovary, epidermis, cuticle, intestine, pseudocoel, and muscle cells. Think about the composition of the body wall: cuticle (noncellular), epidermis (cellular, and muscle cells. The muscle is derived from the mesoderm. From what primary germ layer is the intestine derived ? Can you detect muscle tissue adjacent to the endoderm ? (Atlas p. 112)

3. Trichinella spiralis

The Trichina worm is parasitic in pigs and humans. It is ingested upon consumption of poorly cooked infected meats. The encysted worm is released by digestive enzymes of the host, bores through the intestine wall, and travels in the circulatory system to the skeletal muscle where it once again forms a cyst within the muscle tissue. In this slide you can see the parallel fibers of the skeletal muscle and the spiral Trichina worm within its protective case. Draw and label the encysted Trichina worm in muscle tissue. (Campbell p. 607)

PRESERVED SPECIMENS

1 Ascaris: Obtain a specimen of Ascaris in a dissecting pan but DO NO DISSECTING. Examine the specimens.

A. Identify both male and female organisms. Ascaris exhibits sexual dimorphism. The posterior end of the male is curved or hooked. The posterior end of the female is straight and slightly thicker than the anterior end. (Atlas p. 111)

B. Using a hand lens or dissecting microscope, note the three lips of the Ascaris. There is one dorsal lip and two ventral lips. Sketch the arrangement and label the dorsal and the two ventral lips. Also note the anus located just ventral and anterior to the posterior end. Note the general body form and texture. What is the purpose of the cuticle?

C. Obtain a museum jar containing a dissected Ascaris worm. Note the abundance of coiled tubes packaged within the body wall. What is the relationship between this abundance and the repetition of reproductive organs seen in the intestinal c.s. of Ascaris?

Q1 What features of Ascaris are possible adaptations to parasitic life?

Q2 What are the advantages of having a complete digestive tract?

Q3 What are the advantages of having a body cavity?

PREPARED SLIDES:

1 Phylum Rotifera

This organism is multicellular yet microscopic. It is no larger than the unicellular Paramecium. Rotifers are sometimes called “wheel animals” because of the tufts of cilia (corona), which move continuously around the mouth giving the appearance of a turning wheel. These organisms are pseudocoelomates but are much more anatomically complex than flatworms. (Atlas p. 112; Campbell p.607)

Rotifers exhibit species cell constancy, which means that each adult member of a given species has exactly the same number of cells which is constant for that species. Because of this constancy, there is no opportunity for growth or repair in the adult rotifer. The rotifers also exhibit an unusual reproductive pattern called parthenogenesis.

Q4 Consult your textbook and describe this reproductive phenomenon parthenogenesis.

PHYLUM ANNELIDA (Campbell pp. 612-614)

The phylum Annelida contains coelmate organisms and includes aquatic, terrestrial, and parasitic members. Most aquatic members are marine and have either a free-living or burrowing habit. The earthworm, Lumbricus terrestris, is a common

terrestrial annelid, while leeches are parasitic representatives. The phylum name is derived from the Latin root word annulus, which means “little rings.” This is a reference to the obvious segmentation, which is a distinguishing characteristic of annelids.

PRESERVED SPECIMENS: Observe. DO NO DISSECTING.

1 Nereis – clamworm

Class Polychaeta (many setae). These marine organisms are most often found on the ocean floor where they burrow into the sediments during the day and feed at night. Note the segmentation, the sensory appendages of the head, and the parapodia and setae. What is the literal meaning of the word parapodia? What functions do you think the parapodia and seta serve? (Atlas p. 116; Campbell 614).

2 Hirudo medicinalis – leech

Class Hirudinea. Leeches are parasitic and live on the blood or tissues of their host. What specialized features does the leech exhibit for its specialized parasitic existence? (Atlas p. 118; Campbell p. 614)

3 Lumbricus terrestris

Class Oligochaeta (few setae). You will examine this specimen as you initiate your dissection after examining the microscope slides. (Altas; Campbell p. 613)

PREPARED SLIDES:

1 Earthworm intestinal region c.s.

Label the cuticle, epidermis, circular muscle, longitudinal muscle, dorsal blood vessel, typhlosole, ventral blood vessel. You may have to examine more than one slide to satisfy your recognition of these elements. Note the two layers of muscle tissue, one inside the skin and a second lying on the surface of the intestine. With muscle in these two positions, what kind of coelom does this animal have? (Atlas p. 118; Campbell p. 613)

2 Earthworm setae c.s.

There are four pair of setae in each metamere though the slide that you examine may not show all four pair of setae. The position of the section will not likely hit them all nor is it possible to section without some distortion of the specimen. Label setae.

3 Earthworm nephridiopore c.s.

The earthworm has paired structures, metanephridia, within each metamere except the first. The metanephridia filter the fluid of the coelom and wastes are released through openings in the body wall called nephridiopores. Look carefully to identify both the nephridium and a nephridiopore. Label both.

4 Leech w.m.

Label eyes, mouth, proboscis, testes, ovary, caecum, posterior sucker, and anus. (Atlas p. 118; Campbell p. 614)

& LUMBRICUS DISSECTION: (Work with a partner)

1 Obtain an earthworm specimen and rinse it well under running water. Place the specimen in a dissecting pan.

2 Study the external anatomy and observe the clitellum, the anterior end, the posterior end, the protostomium, metameres, and setae. (Atlas p. 117)

3 Make an incision through the dorsal body wall from the anterior end and continue the incision just past the clitellim. An incision through the body wall made by lifting a sharp pair of scissors will protect the underlying organs. Pressure from a scalpel will destroy the specimen.

4 Observe the organs through the complete digestive tract of the earthworm: mouth, Buccal cavity, pharynx, crop, gizzard, intestine, anus. Most of the length of the digestive tract is intestine which follows the gizzard in the vicinity of the clitellum and continues to the anus. What is the function of the gizzard ? (Atlas pp. 117-118)

5 The closed circulatory system is composed of three components: the dorsal blood vessel, the ventral blood vessel, and the hearts. The singular organ is composed of five paired vessels which circumscribe the pharynx. This is a “landmark” feature within your specimen. Since the earthworm has no respiratory system, how is the position of the dorsal and ventral blood vessels in relation to the body wall significant ? Why do earthworms come to the surface when the ground is rain soaked ? (Atlas pp. 117-118)

6 The nervous system is composed of the superpharyngial ganglion, the subpharyngial ganglion, the circumpharyngial ganglion, the ventral nerve chain. The nerve chain will lie beneath the ventral blood vessel and it will appear as a shinny white thread along the body wall. The nerve chain is a solid structure unlike the dorsal tubular nerve chord possessed by higher animals.

7 The reproductive system is well developed and spread over several metameres. Earthworms are dioecious (hermaphroditic) – that is, they possess both ovary and testes. Earthworms are not, however, self-fertilizing. They are cross-fertilizing receiving sperm in the seminal receptacle and depositing sperm in the cocoon secreted by the clitellum. Eggs are also secreted into the cocoon (Atlas p. 117) which eventually “slides” off the posterior end of the earthworm as the earthworm travels in a forward direction. External fertilization occurs inside the cocoon. The requirement for gametes from another organism, rather than self-fertilization, insures genetic diversity. Observe the seminal vesicles and the seminal receptacles, which appear as small white spheres near the hearts. (Atlas pp. 117-118)

8 The excretory system is segmentally arranged as seen in the paired nephridia in each metamere except the prostomium). The pair of coiled lie on either side of the intestine with a nephrostome on the anterior and the nephriopore on the posterior end which opens through the ventral body wall. The nephrostome drains the fluid of the metamere just anterior to the one in which the nephridium is located. Remember that the excretory system functions to remove nitrogenous wastes from the fluid-filled coelom which result from protein digestion.

9 When you have completed your dissection, dispose of your specimen in the container designated by your instructor. Rinse your dissecting pan, drain completely, and stack for drying.

10 Review the organ-systems using the museum jars of the earthworm dissection. Be prepared to identify internal and external features and the specific organs described above.

PHYLUM ARTHROPODA (Campbell 614-624)

This phylum is the largest in terms of members perhaps because of their success. Evidence indicates that arthropods may have lived on Earth half a billion years ago. There is scarcely a habitat from which insects are absent. They are found in both marine and fresh waters as well as most every terrestrial niche. The phylum includes the trilobites (Campbell p. 617), which are now extinct and the horseshoe crab (Campbell p. 612) which has remained virtually unchanged since its appearance on Earth hundreds of millions of years ago. For that reason, some consider the horseshoe crab to be a living fossil.

PREPARED SLIDES

1 Housefly head

Class Insecta. Observe this insect for arthropod characteristics. Label the compound eye, a facet of the compound eye called an ommatidium, and the antennae.

2 Tick

Class Arachnida. Ticks are arachnids. Unlike insects which have three body regions and six legs, arachnids have just two body regions and eight legs. Observe this organism for arthropod characteristics. (Atlas p. 120)

3 Flea

Observe for arthropod characteristics. Is this organism an insect or an arachnid?

PRESERVED SPECIMENS:

1 Observe the preserved specimen of the horseshoe crab and identify the arthropod characteristics. (Atlas p. 119; Campbell p. 617)

2 Obtain specimens of a terrestrial and an aquatic arthropod: the grasshopper Romalea and the crayfish Cambarus. Place the two organisms in a dissecting pan. DO NO DISSECTING. Observe the organisms for their visible external features. List the arthropod characteristics in common and those unique to each organism. (Just in case you didn’t recognize it, this is one of those compare and contrast opportunities.) Describe these organisms as to body symmetry, supportive structures, appendages, and segmentation. (Atlas pp. 121-122, 125; Campbell 615, 622).

3 Observe the arthropod specimens provided. You will be expected to identify each as arthropods and to name the class of arthropod that each represents. (Atlas pp. 119-125)

Q5 What problems does the exoskeleton of the arthropods eliminate?

PHYLUM ONYCHOPHORA

The walking worm, Peripatus, exhibits some annelid characteristics and some arthropod

characteristics. (Campbell p. 616). They are segmented and possess excretory organs

similar to the annelids. The respiratory and circulatory systems more closely resemble

arthropods. Could Peripatus be the living evolutionary link between the annelids and

the arthropods?

PRESERVED SPECIMENS

1 Peripatus

Observe the specimen and plastomount for both annelid and arthropod characteristics. The dissecting microscope will be helpful here. Record your observations.