CONSIDER THE ANNELIDS
Segmented worms make up the Phylum Annelida. All are bilaterally
symmetrical and their body construction is characterized by a series of
similar, ringlike segments and paired setae (bristles). Annelids
possess a relatively large coelom, a closed circulatory system,
an efficient excretory system and a complete digestive tract. A well-developed
nervous system of ganglia and nerve cords complements a highly developed
muscular system and, compared to other phyla studied thus far, annelids
have an increased concentration of sensory organs at the head end.
The earthworm, Lumbricus, is representative of Class Oligochaeta,
which also includes many other species found mostly in soil and freshwater
habitats. Oligochaetes typically have many segments, but few setae per
segment. The head is reduced, and au possess a clitellum. They are
hermaphroditic, but cross-fertilization is necessary. Use the dissecting
guide (blue handout) and the diagrams in Buschbaum to study the external
and internal anatomy of Lumbricus in lab today.
Class Polychaeta are mostly marine species. The body is also made of many segments and each segment is equipped with a pair of lateral appendages, parapods, with many setae. The parapods are used for both locomotion and respiration. Unlike oligochaetes, most polychaete worms have a well-defined head, no clitellum and are usually dioecious (separate sexes). The nereid or clam worm is a good example of a typical polychaete. Your study of polychaetes in lab is limited to external anatomy, but use a stereoscope to better observe the clam worm head and body.
Leeches, Class Hirudinea, are specialized for parasitism and found mostly in freshwater habitats. The head end consists of a small sucker with a mouth modified for sucking blood from a temporary host. There is also a posterior sucker for attachment. Leeches lack parapods and setae, and though there is external segmentation, the partitions internally and the coelom are much reduced. A chitellum appears during the reproductive season, and, like oligochetes, the leeches are hermaphroditic but need to cross-fertilize with another individual.
Leeches have a long history of use and misuse in medicine. Their value is applied today in treatment of patients who have undergone microsurgery, such as limb reattachment. A short article about medicinal leeches accompanies the leech specimens in the museum display.
KINGDOM ANIMALIA: PHYLA ANNELEIDA
Phylum Annelida
Class Oligochaeta (mostly terrestrial and freshwater species)