phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called stomochord, a structure similar to notochord. This phylum consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals.
The body is cylindrical and is composed of an anterior proboscis , a collar and a long trunk (Figure . ). Circulatory system is of open type. Respiration takes place through gills.
Excretory organ is proboscis gland. Sexes are separate. Fertilisation is external. Development is indirect.
Examples: Balanoglossus and Saccoglossus. . . Phylum – Chordata Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord , a dorsal Figure .
Examples of Echinodermata : (a) Asterias (b) Ophiura Figure . Balanoglossus Proboscis Collar Trunk hollow nerve cord and paired pharyngeal gill slits (Figure . ). These are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation.
They possess a post anal tail and a closed circulatory system. Table . presents a comparison of salient features of chordates and non-chordates. Phylum Chordata is divided into three subphyla: Urochordata or Tunicata , Cephalochordata and Vertebrata .
Subphyla Urochordata and Cephalochordata are often referred to as protochordates (Figure . ) and are exclusively marine. In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life. Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Branchiostoma (Amphioxus or Lancelet) .
The members of subphylum Vertebrata possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates. Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.
Nerve cord Notochord Post-anal part Gill slits Figure . Chordata characteristics Figure . Ascidia T ABLE . Comparison of Chordates and Non-chordates S.No.
Chordates Non-chordates . Notochord present. Notochord absent. .
Central nervous system is dorsal, Central nervous system is