<P> The body surface of trematodes comprises a tough syncitial tegument, which helps protect against digestive enzymes in those species that inhabit the gut of larger animals . It is also the surface of gas exchange; there are no respiratory organs . </P> <P> The mouth is located at the forward end of the animal, and opens into a muscular, pumping pharynx . The pharynx connects, via a short oesophagus, to one or two blind - ending caeca, which occupy most of the length of the body . In some species, the caeca are themselves branched . As in other flatworms, there is no anus, and waste material must be egested through the mouth . </P> <P> Although the excretion of nitrogenous waste occurs mostly through the tegument, trematodes do possess an excretory system, which is instead mainly concerned with osmoregulation . This consists of two or more protonephridia, with those on each side of the body opening into a collecting duct . The two collecting ducts typically meet up at a single bladder, opening to the exterior through one or two pores near the posterior end of the animal . </P> <P> The brain consists of a pair of ganglia in the head region, from which two or three pairs of nerve cords run down the length of the body . The nerve cords running along the ventral surface are always the largest, while the dorsal cords are present only in the Aspidogastrea . Trematodes generally lack any specialised sense organs, although some ectoparasitic species do possess one or two pairs of simple ocelli . </P>

Which are the major 2 groups of flatworms