DIVERSITY OF CHORDATES
 
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MODULE 3: CEPHALOCHORDATA

3.1. Module Objectives

At the end of the lesson the learner will be able to:

3.1.1. Define the Subphylum Cephalochordata.
3.1.2. State the basic characteristics of the Subphylum Cephalochordata.
3.1. 3. Describe the basic characteristics of Amphioxus (Lancet).

3.2. Basic features of the Cephalochordates.

One of the basic features of the Cephalochordates is the presence of a distinct head with mouth = “Cephalo”. The cephalochordates are commonly called lancelets, because they are bladelike in shape. They are small in size, ranging from 3-7 cm long and are are compressed fishlike animals. Cephalochordata are found in marine waters, burried in the sand. The common Cephalochordate is the Amphioxus.

 3.3. Basic features of the Cephalochordates.

Cephalochordates have both larval and adult stages. Both the adult and the larvae.
Cephalochordata possess all the five chordate features- dorsal hollow nerve cord, dorsally situated notochord, laterally situated haryngeal clefts, post anal tail and myortomes.
Cephalochordata are filter feeders, using the gill slits.
Respiration or gas exchange takes place through the body surface.
Cephalochordata possess closed circulatory system- vessels pump” blood by contracting.

3.4. Amphioxus- Branchiostoma.

The common Cephalochordate is the Amphioxus.

3.4.1. Basic characteristics of Amphioxus.

Some of the basic characteristics of Amphioxus are:

Amphioxus have distinct head with mouth = “Cephalo”. They live in marine environments, buried in substrate, with the antherior end exposed on the surface.

Amphioxus looks like fish, but donot have distinct head, jaws, paired fins and no scales.

The body is long, pointed at both ends and compressed. They posses a dorsal fin, caudal fin and ventral fin.

Amphioxus possess segmented myotomes, used for movement.

Amphioxus have several laterally situated pharyngeal gill slits.

Amphioxus are filter feeders, feeding on suspended planktons in the marine environment.

Amphioxus exhibit closed circulatory system.

Figure 1 Amphioxus

3.4.1.2. Amphioxus characteristics shared with vertebrates

In addition to possessing all the five chordate characteristics, Amphioxus shares several features with vertebrates. Some of these features are:

  1. Segmented myomeres (muscle blocks)
  2. Dorsal and ventral aortas
  3. Branchial (gill) arches (blood vessels running over the gills).

3.5.Module summary

You have the following from this module:

  1. Members of the subphylum Cephalochordata are commonly called Lancet, because they are blade-like.
  2. Cephalochordates are found in marine environment, buried in the sand.
  3. The adult possess all chordate characteristics, dorsal hollow nerve cord, notochord, post anal tail, pharyngeal slits and myotomes
  4. The common Cephalochordate is the Amphioxus
  

 

 

 

 

 
 
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