Download 11. phylum annelida

PHYLUM ANNELIDA
There are 15,000 described species in this phylum of vermiform (worm-like) animals.
Earthworms, leeches and marine polychaetes are placed in the taxon. Although some are
parasitic, most annelids are free-living on both land and in the aquatic realm. They are
elongated worms, cylindrical in cross-section and possess muscular body walls. The
feature setting annelids apart from other worm-like animals is segmentation
(metamerism), a serial repetition of both external and internal body parts. Specialization
of body segments for specific functions is an evolutionary consequence of segmentation.
This condition has also evolved in two other animal phyla, Arthropoda and Chordata (to
be studied later). The annelid coelomic cavity is divided into fluid-filled compartments
allowing for increased hydrostatic pressure, a useful adjunct for locomotion, swimming
and burrowing. By shifting coelomic fluid from compartment to compartment, different
pressure can be effected allowing precise movements.
Class: Polychaeta
Polychaetes are characterized by extensions of their body termed parapodia containing
supporting bristles or setae. These structures are employed in locomotion and
respiration. Each parapodium has a dorsal lobe or notopodium and a ventral lobe, the
neuropodium each bearing a bundle of setae and fleshy cirrus. A long chitnous spine
(aciculum) embedded in each lobe lends support during locomotion. Body muscles
attached to the aciculum act on the parapodium much like oars in a boat. There is a
distinct head bearing eyes, palps and tentacles. Sexes are separate (dioecious) in these
predominately marine worms and a larval stage, known as a trochophore, is present.
Polychaetes lack a clitellum (see below).
Nereis is commonly studied in zoology classes and goes by the common names: clam
worm, rag worm, sand worm or mussel worm. This confusing array of names
emphasizes the importance of scientific names to avoid confusion. Nereis are shallow
water marine worms living in mud, sand, debris, crevices and sometimes burrows. They
feed on small invertebrates at night (nocturnal).
External Anatomy. Obtain a worm; place it in a tray and cover with water. The head is
composed of a small prostomium surrounded by the first segment or peristomium. The
prostomium bears a pair of median tentacles, a pair of lateral, fleshy palps, plus four
small dark eyes. Four pairs of tentacles and a ventral mouth can be seen on the
peristomium. In life, hydrostatic pressure forces the pharynx, bearing small horny teeth,
out of the mouth facilitating prey capture. Gently squeeze the worm behind the head to
evert the pharynx. The posterior-most segment of Nereis is termed the pygidium and
bears the anus.
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Figure 1. Draw and label a cross section of a parapodium 5 cm in diameter from
a slide.
Study Questions
1.
2.
Are all parapodia the same?
How does the parapodia function in respiration?
Class: Oligochaeta
Oligochaetes lack parapodia and have a reduced number of setae. A clitellum for
producing a cocoon to receive eggs is present. Members of this class are usually
monoecious. (hermaphroditic) The head is poorly developed and lacks appendages.
Development is direct (no larval stage). Lumbricus terrestris , the earthworm, is the
subject of the laboratory study. These nocturnal soil dwelling herbivores are worldwide
in their distribution.
External Anatomy. Locate the four anteriormost segments representing the head. The
first segment is the peristomium and bears the mouth. Just above the mouth is the small
prostomium. There are no specialized sense organs on the head. Male pores (on the
ventral aspect) open on segment XV, while female pores (ventral) can be seen on
segment XIV. The seminal receptacle is located in grooves between segments IX-X and
X- XI. but may be difficult to locate. Now locate the clitellum on segments XXXII
through XXXVII. The last segment or pygidium, bears the anus.
Internal Anatomy. Pin your worm in a dissection tray dorsal (darker) side up. Place a
pin in the peristomium and in the clitellum. Carefully make a shallow cut from the head
to the clitellum. Now carefully cut each septum and pin both sides of the worm along the
length of the incision. Each septum surrounds the body and forms a separate
compartment.
Locate the three pairs of light colored seminal vesicles attached to segments IX-XI,
These sperm sacs hold spermatozoa as they mature. (Two pairs of small-branched testes
are housed in the seminal vesicles and two tiny ducts connect the testes with the male
pore). Find the female seminal receptacles on segments IX and X, they store sperm
after copulation with another worm. Search and you may find a pair of ovaries under the
third pair of seminal vesicles and the paired oviducts carrying eggs to the female
opening.
Beginning with the mouth, locate the pharynx attached to the body by muscles that
produce a sucking action. The esophagus extends from segment VI to XIII and is
partially hidden by the dark colored hearts and the seminal vesicles. A large, thin-walled
crop, used for food storage, is on segments XV and XVI. Adjacent to the crop (on
segments XVII and XVIII) lies the muscular gizzard utilized for grinding food. From
here the intestine, for digestion and nutrient absorption, extends to the anus.
Circulation in annelids is a closed system with blood flowing continuously in vessels
rather than into body spaces (sinuses). Locate the five pairs of hearts between segments
VII and XI and the dorsal and ventral blood vessels running the length of the body.
Examine a cross-section of an earthworm and try to locate a pair of tubular nephridia.
Each nephridium has a ciliated funnel or nephrostome connecting to the anterior septal
wall of each segment. The nephrostome draws wastes from one segment into the ciliated
tubules of the next posterior segment where it is excreted via the nephridiopore.
Figure 2. Using the outline provided in your lab manual, draw the digestive
system and the reproductive system of the earthworm. Label the anatomy and
locate the structures in the correct segments. Make sure to include Roman
numerals for each segment on you work sheet.
Figure 3. From a prepared slide draw a x. s. of a segment posterior to the
clitellum. Label the excretory structures, the ventral nerve cord and other
observable anatomy.
Class: Hirudinea
Leeches lack parapodia and setae but possess a clitellum. Anterior and posterior
suckers are present and the body has 33 or 34 segments. Minute annuli (1 to 5 per
segment) should not be mistaken for additional segments. Hirudineans reside in aquatic
and terrestrial habitats. Leeches are carnivores or blood suckers (ectoparasites). Hirudo
is a European genus found in freshwater and used for blood letting for the past 1000
years.
Figure 4. Draw a leech 10 cm long. Label external anatomy.