The sub-phylum Cephalochordata includes a single class Leptocardii, which has single family, Branchiostomidae. The family contains only two genera Branchiostoma and Asymmetron.
The subphylum - Cephalochordata includes Amphioxus or Branchiostoma.
- Pallas described this animal first. He named it as Umax lancealatus, He included this in Mollusca.
- In 1834, costa described its chordata features.
- In 1836, Yarrel named it as Amphioxus.
CHORDATE FEATURES OF CEPHALOCHORDATA (AMPHIOXUS)
- Presence of dorsal tubural nerve cord.
- Presence of a long notochord from anterior end to posterior end on the dorsal side. Because, it extends to the cephalic region. Hence it is called Cphalochordata.
- Gill silts are present in the
- Presence of post anal tail.
- The presence of liver diverticulum.
- The development of hepatic portal system.
- Presence of Myotomes and which are useful for locomotion.
- The presence of dorsal, caudal and ventral fins.
In these characters Cephalochordata resembles chordates.
PRIMITIVE CHARACTERS OF CEPHALOCHORDATA
- The excretory system contains protonephrldia.
In the chordate world, the presence of solenocytes is not reported. But, in Amphioxus solenocytes are associated with Nephridium.
- Absence of heart and kidney.
- Absence of paired limbs or paired fins.
- Absence of distinct head.
- Absence of distinct paired sense organs.
- Gonads are without gonoducts.
Thus cephalochordates will show many primitive characters.
SPECIAL CHARACTERS OFCEPHALOCHORDATA
- Because of its ciliary mode of feeding, the pharynx is elaborated with many gill slits.
- Oral hood is well developed for ciliary mode of feeding.
- Because of its ciliary mode of feeding the atrium is very well developed.
Thus, Amphioxus shows some special characters which are developed because of its ciliary mode feeding.
CEPHALOCHORDATES RELATIONSHIP WITH UROCHORDATES
Cephalochordates show many close relationship with urochordata. In some points they differ.
- Presence of gill slits In pharynx.
- Presence of Endostyle in Pharynx.
- Presence of Ciliry mode of Feeding.
- Presence of atrium.
- Absence of test.
- Absence of distinct heart
- Presence of notoçhord and nervecord in the adults.
- Presence of Myotomes in the adult.
In this way Cephalochordates differ with Urochordates.
Cephalochordates show some invertebrate features, They are,
- Presence of paired nephridia like annelids.
- Presence of flame cells like Helminths.
- Presence of soft body and slug like appearance like molluscans.
By considering the above facts we come to a conclusion that Amphioxus is a chordate animal. Amphioxus is a degenerate jawless chordate animal. It mainly shows chordate features. It differs from urochordates in some aspects. Hence, It is separated and kept in a separate sub phylum called Cephalochordate.