Animals have three basic body plan as given below
(i) Cell aggregate body plan Cells are loosely arranged
and do not organise to form tissues, e.g., sponges.
(ii) Blind sac body plan Body has a cavity that helps in
digestion with a single opening, the mouth, e.g.,
coelenterates.
(iii) Tube within tube body plan It is found in higher
animal groups from nematodes to chordates. The body
consists of two openings, i.e., one for ingestion and
one for egestion.
It is further divided into two types
(a) Protostomous plan (Proto-first; stoma—mouth)
found in non-chordates like flatworms,
roundworms, annelids, molluscs and
arthropods. In this, mouth is derived from the
blastopore of the embryo and anus is formed
later.
(b) Deuterostomous plan It is found in
echinoderms, hemichordata and chordates. In
this, anus is derived from the blastopore and
mouth arises at the opposite end.