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Phylum: Annelida - [little ring] (segmented worms) earthworms, marine worms, leeches. There are
about 15,000 species of annelids, most of which live in marine
habitats. Annelids are characterized by having elongate, conspicuously segmented
body. Unlike the molluscs with their
reduced coelom, annelids have a large coelom that is divided by septa between
individual segments.
Longitudinal section of
earthworm Extreme segmentation is
believed to be an adaptation for burrowing in soil. Annelids manipulate
the fluid filled coelom in each segment to
move. When circular muscles contract, the segment takes on a long
skinny shape. When longitudinal muscles contract, the segment becomes
short and stout. When the segment becomes short and stout, the setae
(spines) push out and grip the wall of the burrow. Anyone who has
tried to pull an earthworm from the ground can attest to the
effectiveness of the setea in gripping the
soil. By manipulating segments in different areas of the body the
annelid can push its way through the sediment. Many aquatic forms will
swim using an undulating motion. Small extensions of the body called parapodia act as little wings and aid in
swimming as well as funtioning in
respiration. The organ systems of annelids are relatively complex and show much specialization in the digestive tract. The circulatory system is closed with a blood vessel running along the dorsal and ventral surface of the digestive tract. Five aortic arches (hearts) are located laterally along the esophagous and connect the dorsal and ventral blood vessels.
Cross-section of an
earthworm. Excretion in annelids is accomplished by a pair of metanephridia in each segment. Unlike protonephrida which have only one opening (see Hickman et al. fig 14-9), metanephridia have two openings (see fig 17-18). One opening takes in coelomic fluid and the other end releases excretory filtrate outside the body. As the fluid flows through the metanephridia, water and salts are reabsorbed from the excretory filtrate. Trochophore larvae. Annelids can reproduce both asexually and sexually. Asexual reproduction by fragmentation is common. Individuals are monoecious but cross-fertilize, i.e., each individual transfers sperm to the mate. Young in marine species develop into a trochophore larvae. Classes of Annelids:
Phylum Mollusca [soft body] snails, bivalves, squid and octopi. Phyla Mollusca
are protostomes that produce trocophore type larvae during development.
Recent phylogenetic evidence indicates
that Molluscs and Annelids are closely
related. The typical mollusk has
three body components: head-foot, visceral mass, and mantle. The head-foot
contains sensory organs (head) and is used for locomotion (foot). The visceral
mass contains the organs for most major systems (digestive,
reproductive, circulatory, etc.). The mantle
surround and protects the body and in some species, secretes a
hard calcareous shell. Bivalves produce pearls when foreign
debris gets between mantle and shell.
Clam anatomy Reproduction in molluscs is sexual with individuals being dioecious. One of the more interesting
reproductive characteristics is the glochidial
stage of development in some freshwater mussels. The glochidia larvae attach to the
gills or fins of fishes and hitch a ride for about 2 weeks until they
detach and settle to the substrate. This behavior allows some mussels
to be transported upstream to colonize new mussel beds. A- Clam
glochidia, B- Female with mantle lure. Molluscs have bilateral symmetry and typically have a short body with a very small coelom. Trying to characterize molluscs is difficult because of the different levels of complexity seen in body form and sensory organs of the classes. Classes Polyplacophora and Bivalvia live relatively sedentary lives and do not have well-developed sensory organs. Class Cephalopoda contains active predators with a high degree of cephalization. The cephalopods are the most intelligent invertebrates and have a well-developed brain and sensory organs. The eye of cephalopods is highly developed and, similar to the vertebrate eye, can judge distances. Although most molluscs have an open circulatory system, cephalopods have a closed system. The body of snails have undergone body twisting "torsion" to accommodate the single shell. Torsion twists the digestive system so the anus opens just over and behind the head, only one lung and one kidney is present. Classes of Molluscs: |
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