AMMOCOETES LARVA

Ammocoetes larvae-Matamorphosis

In the life history of Petromyzon a larval form is seen. It is called Ammocoetes larva.

1) Ammocoetes larva is transparent and 1 cm., in length.

2) Ammocoetes larva lives in U shaped burrows of mud.

3) Ammocoetes larva comes out of the burrow at night only to change its feeding ground, then it buries again.

4) Ammocoetes larva has an eel like body and it differs from the adult In several characters.

5) It has no buccal funnel.

6) Around the mouth a semi circular oral hood is present

7) Below the mouth short transverse lower lip is present and it has no teeth.

8) Paired eyes are covered by thick skin.

9) Behind the buccal tentacles, a velum with a pair of cup-shaped muscular flaps are seen.

10) Surrounding the mouth buccal tentacles are present.

11) The pharynx is associated with seven pairs of gill pouches.

12) The pharynx is continued into the oesophagus posterioily.

13) The endostyle is present on the ventral side of the pharynx.

14) The mucus secreted by the endostyle passes into the peripharyngeal grooves of the pharynx.

15) Food particles like unicellular algage, bacteria which enter with the incurrent of water and entangled with the mucous. Thus the food associated with mucous carried into the oesophagus from the pharynx. Because of muscular movement of velum and pharynx, the ammocoetes larvae feeds.

 

Metamorphosis of Ammocoetes larva

1) After a long period of larval life, the Ammocoetes larva metamorphoses into the adult.’

2) The endostyle modifies into the thyroid gland.

3) The oral hood changes into the buccal funnel with horny teeth,tongue and a round mouth.

4) Paired eyes are developed completely.

5) The velum degenerates by leaving a rudiment.

6) The continuous dorsal fin breaks into dorsal and a single caudal fin.

7) The young lamprey migrates to the open sea from rives and becomes carnivorous adult animals.

The ammocoetes larva exhibits striking similarities with Amphioxus. it is a very primitive and generalised vertebrate. It may be regarded as the connection link between Amphioxus and cyclostomes.

BRANCHIAL BASKET OF PETROMYZON

Branchial Basket

In Petromyzon the visceral skeleton contains branchial basket. It lies external to the gill pouches. It shows 9 irregular vertical bars and 4 longitudinal bars on either side. The first vertical bar lies just behind the styloid cartilage. The second is in front of the first gill slit. The remaining seven vertical bars are behind the 7 gill slits. The last vertical bar Is joined to a cup like pericardial cartilage. It supports the posterior and lateral walls of the pericardium. The longitu1nal bars are called subchordal, epitrematic, hypotrematic and median ventral. The subchordal bar runs along the notochord. Ft is joined to the cranium In front. The epitrematic and hypotrematic bars lie above and below the gill slits. The median ventral bar is placed in the midventral line and partly fused with the corresponding bone of the other side.

 

Branchial Basket-Function:

In Petromyzon branchial basket give support to the gills. It will keep the gill slits open throughout.

PETROMYZON-MYXINE-DIFFERENCE

Myxine (Hagfish)		Petromyzon (Lamprey)
I Hag fishes are scavengers. They penetrate into the dead fishes bodies and eat the tissues.		1.They are semi parasitic. But the Larvae(Ammocoetes) leads an independent life.
2. The body is. slender & eel-like		2. The body is cylindrical & stout.
‘3 They are exclusively marine habitats.		3. These live in both in sea water and fresh water. They migrate into the rivers for spawning.
4. the caudal fin is small.		4. The caudal fin is well developed.
5. The dorsal fin is not fully developed.		5. The dorsal fin is divided into two portions by a notch.
6. Mouth is terminal in position. It is surrounded by soft lips. Buccal funnel is absent.		6. Mouth is sub-terminal in position. Buccal funnel is present.
7. Single nostril is present very near to the mouth.		7. Single nostril Is on the dorsal side of head region.
8. Skull has no roof.		8. Skull roc is incomplete.
9. Small teeth are present on the tongue in two rows.		9. Horny teeth are present in the buccal funnel and on the tongue.
10. Salivary glands are present.		10. Paired salivary glands are present
11. The gill pouch number varies from six to fifteen pairs.		11. There are seven pairs gill- pouches present. All these open Into the respiratory tube.
12.Each aortic ‘arch supplies the hemibranch of a single gill pouch.		12. Each aortic arch divides and the branches enter the posterior hemibranch of the nearby gill pouch.
13. Brain is degenerated.		13. Brain is well developed.
14. Only thee pairs of cranial nerves are present.		14. Ten pairs of cranial nerves are present
15. Non functional paired eyes are burned in the skin.		15. Paired functional eyes are present.
‘16. Penial eye is absent.		16. Penial eye is present.
17. In the adult pronephros is retamed. But the functional kidney Is of mesonephric type and simple.		17. The functional adult kidney is mesonephric type.
18. UrinogenitaL sinus is not seen:		18. Urinogenital sinus is formed.
19. Sexes are united - Hermaphrodite. The anterior part of the gonad acts as ovary & the posterior part act Like Testis.
20. The eggs are large and covered by cylindrical horny shell,		20. The eggs are small and are not covered by horny shell.
21. The development is direct		21 Development is indirect. Arnmocoetes larva occurs in the life-history.

 

 

CYCLOSTOMATA-CLASSIFICATION

classification of cyclostomata

The class Cyclostomata is divided into two orders:

 

1) Petromyzonia and 2) Myxinoidea

.

Order 1. Pefromyzonia

1)Lampreys are included In this order.

2) The buccal funnel is suctorial and shows horny teeth.

3) The mouth is present in the buccal funnel.

4) The nasal sac is dorsal. It has no connection with the pharynx.

5) Eyes are functional.

6) Seven pairs of gill slits are present.

7) A well developed dorsal fin is present.

8) Branchial basket is complete.

9) Brain is well developed.

10) Pineal eye is well developed.

11) Ear has two semicircular canals.

Ex: 1. Petromyzon (Sea-lamprey) ) Lampetra fluviatilis.

Order 2: Myxinoidea

1) It includes the hag-fishes or slime eels.

2) Buccal funnel is absent.

3) The nasal sac open into pharynx, through a canal.

4) Eyes are vestigial organs.

5) Dorsal fin is absent or very small.

6) Branchial basket is poorly developed.

7) Brain is primitive.

8) Pineal eye is reduced.

9) Ear has only one semicircular canal.

10) The hag-fishes are all marine.

Ex: 1. Myxine glutinosa Hag-fish or slime eel). 2. Eptatretus (Bdellostoma)

 

previous topic-Cyclostomata-characters

CYCLOSTOMATA -GENERAL CHARACTERS

 

cyclostomes-General characters

Cyclostomes’ are jawless vertebrates, They are primitive vertebrates. The cyclostomes are marine or fresh-water vertebrates.

It includes the lampreys and hag fishes.

1) The body is long, eel like. It shows trunk and compressed tail.

2) Paired fins are absent. Median fin is supported by cartilaginous fin-rays.

3) The skin Is soft and smooth. It is slimy. It as scale less.

4) Z shaped myomeres are present in the trunk and tail Protractor and retractor muscles move the tongue.

5) In this group a true coelome is seen.

6) These vertebrates will not have jaws, hence called Agnatha.

7) The mouth is circular. It works like a sucker. It is surrounded by tentacles.

8) Tongue bears teeth.

9) Stomach is absent and oesophagus leads into the intestine.

10) Endoskeleton is present.

11) Skull is simple and primitive.

12) Notochord presists throughout life.

13) Vertebrae are represented by neural arches, around the notochord.

14) Five to sixteen pairs of gills are present in sac like pouches

15) The heart is two chambered. Sinus venousus is present, but conus arterlcsus is absent.

16) Blood contains leucocytes and Irregular nucleated erythrocytes..

17) Brain is seen.

18) Ten pairs or less number of cranial nerves are present.

19) Nasal sac is single and median.

20) Lateral line sense organ is present.

21) Excretory system includes a pair of mesonephric kidneys.

22) Sexes are separate.

23) Gonad is single and without a gonoduct.

24) Development may be direct or with a long larval stage.

 

know more about Amphioxus structure