Question

All vertebrates are chordates, but not all chordates are vertebrates. Compare the three subgroups hemichordata, cephalochordata, urochordata and explain the differences i) between the groups and ii) between those phyla and the vertebrates, iii) and describe the relationship of the hemichordates to other deuterostomes. Finally, iv) What physical feature found in all three groups is an advantage that led to the “pre-adaptedness” found in chordates?

Answer 1

Answer:

(i) Comparison and differences between the three groups:

Properties	Hemichordata	Cephalochordata	Urochordata
Arrangement of notochord and nerve cord	Lack notochord and true dorsal nerve cord	Long notochord and nerve cord	Stiff notochord and dorsal hollow nerve cord
Metamerism	Non-metameric	Metameric	Non-metameric
Shape	Worm-like	Fish-like	Bag-like
Reproduction	Separate sexes	Separate sexes	Hermaphroditic
External body cover	Not covered by any exoskeleton	Not covered by any exoskeleton	Covered by tunic or test

(ii) Both vertebrates and these phyla (except Hemichordata) have notochord and nerve cord, along with pharyngeal gill silts, post-anal tail at some point of their lifetime. But the main difference is that, vertabrates possess a vertebral column which protects the notochord and nerve cord but the chordates do not have any vertebral column.

(iii) In Hemichordates, there is a deuterostome-specific genomic cluster which consists of four order transcription genes. Development of pharyngeal gill silts, which is consisdered as the first morphological innovation of early deuterostomes, is attributed to this genomic cluster found in Hemichordates.