Summarizing Superphylum Lophotrochozoa

Summary

Phylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism. Annelids are protostomes. These animals have well-developed neuronal and digestive systems. Some species bear a specialized band of segments known as a clitellum. Annelids show the presence numerous chitinous projections termed chaetae, and polychaetes possess parapodia. Suckers are seen in order Hirudinea. Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism. Internal segmentation is absent in class Hirudinea.

Flatworms are acoelomate, triploblastic animals. They lack circulatory and respiratory systems, and have a rudimentary excretory system. This digestive system is incomplete in most species. There are four traditional classes of flatworms, the largely free-living turbellarians, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. Trematodes have complex lifecycles involving a molluscan secondary host and a primary host in which sexual reproduction takes place. Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts.

The rotifers are microscopic, multicellular, mostly aquatic organisms that are currently under taxonomic revision. The group is characterized by the rotating, ciliated, wheel-like structure, the corona, on their head. The mastax or jawed pharynx is another structure unique to this group of organisms.

The nemertini are the simplest eucoelomates. These ribbon-shaped animals bear a specialized proboscis enclosed within a rhynchocoel. The development of a closed circulatory system derived from the coelom is a significant difference seen in this species compared to other pseudocoelomate phyla. Alimentary, nervous, and excretory systems are more developed in the nemertini than in less advanced phyla. Embryonic development of nemertine worms proceeds via a planuliform larval stage.

Phylum Mollusca is a large, marine group of invertebrates. Mollusks show a variety of morphological variations within the phylum. This phylum is also distinct in that some members exhibit a calcareous shell as an external means of protection. Some mollusks have evolved a reduced shell. Mollusks are protostomes. The dorsal epidermis in mollusks is modified to form the mantle, which encloses the mantle cavity and visceral organs. This cavity is quite distinct from the coelomic cavity, which in the adult animal surrounds the heart. Respiration is facilitated by gills known as ctenidia. A chitinous-toothed tongue called the radula is present in most mollusks. Early development in some species occurs via two larval stages: trochophore and veliger. Sexual dimorphism is the predominant sexual strategy in this phylum. Mollusks can be divided into seven classes, each with distinct morphological characteristics.

Glossary

Annelida

phylum of vermiform animals with metamerism

captacula

tentacle-like projection that is present in tusks shells to catch prey

clitellum

specialized band of fused segments, which aids in reproduction

conispiral

shell shape coiled around a horizontal axis

corona

wheel-like structure on the anterior portion of the rotifer that contains cilia and moves food and water toward the mouth

ctenidium

specialized gill structure in mollusks

mantle

(also, pallium) specialized epidermis that encloses all visceral organs and secretes shells

mastax

jawed pharynx unique to the rotifers

metamerism

series of body structures that are similar internally and externally, such as segments

Mollusca

phylum of protostomes with soft bodies and no segmentation

nacre

calcareous secretion produced by bivalves to line the inner side of shells as well as to coat intruding particulate matter

Nemertea

phylum of dorsoventrally flattened protostomes known as ribbon worms

parapodium

fleshy, flat, appendage that protrudes in pairs from each segment of polychaetes

pilidium

larval form found in some nemertine species

planospiral

shell shape coiled around a vertical axis

planuliform

larval form found in phylum Nemertea

radula

tongue-like organ with chitinous ornamentation

rhynchocoel

cavity present above the mouth that houses the proboscis

schizocoelom

coelom formed by groups of cells that split from the endodermal layer

seta/chaeta

chitinous projection from the cuticle

trochophore

first of the two larval stages in mollusks

veliger

second of the two larval stages in mollusks