Which of the following options is not a mollusca?

Mollusca: In the animals of this group, there is bilateral symmetry (the left and the right halves of the body are same in design) with little segmentation. The coelom (body cavity) is reduced. They have an open circulatory system (blood does not flow in blood capillaries) and bear kidney-like organs for excretion of waste material. There is a foot, which is used for moving, e.g. unio, chiton, pila, octopus, etc. 

Prawn: Prawn belongs to class arthropoda (Arthro – jointed, poda – legs). Arthropoda is the largest group of animals. These animals are bilaterally symmetrical and segmented. There is an open circulatory system (blood does not flow in well-defined blood vessels) and therefore, the coelomic cavity is filled with blood. Examples are prawns, butterflies, crabs, houseflies, spiders and scorpions, etc.