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.,
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) Deutgrostomous plan It is found in echinoderms, hemichordata and chordates. In this, anus is derived from the blastopore and mouth arises at the opposite end.