Phylum Mollusca and Phylum Annelida
Phyla Mollusca and Annelida are both protostomes that produce trocophore type larvae during development. Recent phylogenetic evidence indicates that they are closely related. Molluscs and annelids also share a eucoelomate body cavity; i,e,. the coelom is completely lined with mesoderm tissue. A major difference between them is that annelids are segmented and have a closed circulatory system whereas molluscs are not segmented and have an open circulatory system.
Phylum: Mollusca -
[soft body] snails, bivalves, squid and octopi.
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.
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.
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 ogans 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:
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.
Extreme segmentation 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.
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 14-6), metanephridia have two openings (see fig 17-14). 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.
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 trocophore larvae.
Classes of Annelids: