Protostomia Deuterostomia Blastopore forms future mouth and anus Blastopore forms future anus only. Mouth forms later. They share similar sequences in their 18S rRNA genes. Planaria share with the other members of the phylum a flat, almost ribbonlike, shape bilateral symmetry. The bilateral symmetry of planarians is associated with active locomotion by secreting a layer of mucus underneath them and propelling themselves forward with the myriad of cilia on their ventral surface.
Planarians feed through a mouth on their ventral surface.
It leads to an elaborate gastrovascular cavity. But there is no separate exit so undigested food has to leave by the mouth. All of these have at least two different stages in their life cycle, each parasitic in a different host - one of which is usually a snail. The diagram gives the life cycle of the blood fluke, Schistosoma mansoni . Once within the alternate host, a snail, a single miracidium may produce as many as 200,000 infectious cercariae.