Superphylum Lophotrochozoa: Molluscs and Annelids

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Describe the morphology and anatomy of mollusks.


Mollusks have a large muscular foot that may be modified in various ways, such as into tentacles, but it functions in locomotion. They have a mantle, a structure of tissue that covers and encloses the dorsal portion of the animal, and secrete the shell when it is present. The mantle encloses the mantle cavity, which houses the gills (when present), excretory pores, anus, and gonadopores. The coelom of mollusks is restricted to the region around the systemic heart. The main body cavity is a hemocoel. Many mollusks have a radula near the mouth that is used for scraping food.

What are the anatomical differences between nemertines and mollusks?


Mollusks have a shell, even if it is a reduced shell. Nemertines do not have a shell. Nemertines have a proboscis; mollusks do not. Nemertines have a closed circulatory system, whereas Mollusks have an open circulatory system.

How does a change in the circulatory system organization support the body designs in cephalopods compared to other mollusks?


Cephalopods have a closed circulatory system, while other members of the Mollusca phylum have open circulatory systems. Having a closed system allows blood to be moved more efficiently and rapidly through the animal, since the circulation is not limited by diffusion. For example, this allows the octopus to have a much more complex body plan, with branching tentacles, compared to a snail. In many cases, a closed circulatory system also allows the development of larger organisms.