Basic Concept of Invertebrate Zoology Knowledge: Description & Types of Echinoderm Larva

Description :-

In Echinoderms the development may be direct or indirect. In direct development is no larva. Direct development is seem in only a few echinoderm living in Arctic & Antarctic waters. In indirect development the life cycle includes one or more larvae. Many types of larvae occur in echinoderms. They are the following-

(1) Dipleurula Larva :-

This larva develops from gastrula. It has the following salient features-

(a) Dipleurula is the fundamental larva of all echinoderms.

(b) It is microscopic.

(d) It is bilaterally symmetrical and oval in shape.

(e) It has a gut formed of the mouth, the oesophagus, the intestine, the stomach & the anus.

(f) It gas two ciliary bands, viz a pre-oral band around the mouth and an adoral band inside.

(g) The pre-oral band is used for locomotion & adoral band is used for the collection of food.

(h) It feed on diatoms.

(i) The dipleurula develops into Bipinnaria larva.

(2) Bipinnaria Larva :-

It is the second larva of Starfish. It develops from dipleurula larva. It has the following salient features-

(a) It is minute, microscopic and swims freely on the water surface.

(b) It is bilaterally symmetrical and has alimentary canal with a mouth of the anterior end and an anus at the posterior end.

(c) The body has a number of outgrowths called arms. The arms are covered by ciliated bands and are used for locomotion. It has two unpaired arms and five pairs of paired arms. They are-

(i) Median dorsal arms

(ii) Median ventral arms

(iii) Pre-oral arms

(iv) Post-oral arms

(v) Antero dorsal arms

(vi) Postero dorsal arms

(vii) Postero lateral arms

(3) Brachiolaria Larva :-

It is the third larva of after a short free swimming life. The Bipinnaria larva. the Brachiolaria larva. The pre-oral region of this larva has three processes called Brachiolar arms. These three arms are tipped with suckers. It swims and feed like a Bipinnaria.

(4) Ophiopluteus :-

(a) It is the larva of Ophiuroidea.

(b) The pre- oral lobe is small.

(d) The arms are supported by Calcareous rods.

(e) The larva has a pair of pre-oral arms, a pair of post oral arms, a pair of postero dorsal arms and a pair of postero lateral arms.

(f) The postero lateral arms are always longer and directed forward, so that the larva appears like a "V" shaped.

(5) Echinopluteus :-

(a) It is the larva of Echinoidea.

(b) It has small pre-oral lobe & a single ciliary band,

(d) The larva is provided with a pair of pre-oral arms, a pair of post-oral arms, a pair of antero lateral arms, a pair of antero dorsal arms & a median posterior arm.

(e) The postero lateral arms are very short & directed backwards.

(6) Auricularia :-

(a) It is the larval form of Holothuroidea.

(b) There is a well developed pre-oral lobe.

(c) Arms are supported not by calcareous rods but the calcareous structures are in the form of wheels, spheres, star shaped bodies etc.

(7) Doliolaria of holothuroidea (Pupa):-

(a) In holothuroidea (sea-cucumber) the auricularia larva develops into doliolaria larva.

(b) The larva is also called a pupa.

(d) It is barrel shaped.

(e) The calcareous skeleton is in the form a spheres.

(f) The ciliated bands are broken into pieces.

(g) Metamorphosis begins during free swimming life. After metamorphosis, the larva sinks into the bottom to become the adult.

(8) Doliolaria of Crinoidea :-

(a) It is the larva of Antedone.

(b) Is is a free-swimming larva.

(d) It is barrel shaped.

(e) It has four or five ciliary bands.

(f) At the anterior end the ectoderm thickens to form an apical plate.

(g) The apical plate bears a tuft of cilia called apical sensor tuft.

(h) An adhesive pit is present between the first and second ciliary bands. It is used for attachment.

(i) A mouth is located between the second and third ciliary bands.

(9) Pentacrinoid larva :-

(a) It is the second larva of antedone.

(b) It develops from Doliolaria larva.

(d) It has a stalk.

(e) the stalk develops from the pre-oral lobe of Doliolaria.

(f) One end of the stalk is attached to the substratum with the help of a disc and the free ends bears a crown.

(g) The crown consists of a central mouth surrounded by a circle of tentacles.

(h) During metamorphosis the crown develops cirri & breaks off from the stalk as a free living antedone.

Metamorphosis :-

During Metamorphosis the bilaterally symmetrical larvae become transformed into radially symmetrical adults.

In all these larval forms during metamorphosis there is alternation of the position of the mouth.

In the Crinoidea, the mouth is shifted to the posterior end. But in all other echinoderms mouth is shifted to the left side with the corresponding shifting of the coelomic cavity.

In Crinoidea and Asteroidea the larva becomes fixed to some substratum by the pre-oral lobe at the time of metamorphosis & a fixation disc is formed for this purpose. In the Crinoidea the fixation persists until the adult form is completely formes. But in Asteroidea fixation is temporary.