Question

Which is not a vertebrate?
(a) Fish
(b) Amphioxus
(c) Oryctolagus
(d) Sparrow

Answer 1

Hint: This is a fish-like animal having a spindle body, laterally compressed and pointed at both the ends. This is whitish to creamy yellow in colour fish but without eyes.

Complete step by step answer:
- Amphioxus shows distinct chordate features throughout life.
- The body is divided into:
The anterior trunk which bears three openings mouth, atriopore and anus
Posterior tail
- The exoskeleton is absent but the endoskeleton is present which is neither bony nor cartilaginous.
- Notochord is the chief axial endoskeleton, structurally it is elongate, narrow, cylinder and rod- like are present.
- They show true coelomate where coelomate is filled with coelomic fluid.
- Their gills slits directly open into atrium and space is formed due to metapleural folds one on either side of gill slits of the embryo.
- Amphioxus has a closed circulatory system with colourless blood due to lack of blood pigment.
- They show the absence of heart and blood is present in the blood vessel and lymph vessel.
So, the correct is, ‘(c) amphioxus.’

Additional information:
- Amphioxus was discovered by Pallas in 1778.
- This belongs to the family Branchiostoma found in the shallow water of the sandy coast in tropical and temperate regions.
- The presence of a true head is missing but it bears three longitudinal fins that are median dorsal fin, median ventral fin and the caudal fin.

Note:
- As amphioxus don’t have eyes but they have eyespots or ocelli which are photoreceptors distributed on the ventrolateral sides of the nerve cord. These are made up of two types of cells: an outer pigments cell or melanocyte and an inner photosensitive cell.
- Amphioxus lacks kidney thus, the excretion occurred through solenocytes which closely resemble the protonephridia of flatworm or annelids showing parallel evolution.