Classification of Subphylum Cephalochordata

In this article we will discuss about the classification of subphylum cephalochordata.

Historical Resume of Subphylum Cephalochordata:

The first individual of this group was des­cribed by Pallas in 1778. He regarded the speci­men as a slug and named it Umax lanceolatus. Yarrell (1836) studied the animal in detail and gave the name Amphioxus lanceolatus. Costa (1834) suggested the name of the genus as Branchiostoma. Since then the biological name of the genus has been established as Branchiostoma according to the law of priority.

Important Features of the Subphylum Cephalochordata Lankester, 1877:

[Gk. Kephale, a head], Approx. 23 species.

a. Cephalochordates are small, fish-like translucent marine chordates.

b. The body is laterally compressed and tapered at both ends with a post-anal tail. They are commonly called “lancelets” for the shape of the body.

c. There is a low continuous dorsal and cau­dal fin; no paired fins.

d. Body muscles are arranged as a series of V-shaped blocks of striated muscles fibres running throughout the body, called the myotomes (myomeres) and are separated by sheets of connective tissue, the myosepta or called myocommas.

e. Epidermis is single layered.

f. The persistent notochord extends from the tip of the tail to the region beyond the brain, hence called Cephalochordata.

g. Ventral mouth surrounded by small tenta­cles, leads into large pharynx with nume­rous gill-slits open into the atrium.

h. Blood vascular system is closed type and lacking of a specialized heart. The blood is colourless and without any respiratory pigment.

i. The dorsal hollow nerve cord runs imme­diately dorsal to the notochord and opens to the exterior through an anterior neuro­pore.

j. The excretory organs are protonephridia with solenocytes, derived from ectoderm.

k. Ductless gonads are metamerically arranged.

l. Sexes are separate (dioecious). Fertiliza­tion takes place in sea-water.

m. Radial cleavage.

n. Development with a transparent asym­metrical planktotrophic larva is similar to the shape of an adult.

Fossil History:

The oldest known cephalochordates is Pikaia (Fig. 3.22), recorded from the Burges Shales of Canada, about the middle Cambrian period. The anatomical characteristic features are more or less same as Branchiostoma except a pair of sensory tentacles that are found at the end of the body.

Geographical Distribution:

Cephalochordates are found near shore areas (sub-tidal areas) of the seas of temperate and tropical regions.

Size:

50 to 70 mm.

Habit and Habitat:

They are the inhabitants of coarse sands, shell gravels and fine sands of the seas. In coarse sand and gravel they will bury com­pletely, but in finer sandy areas the anterior end protrudes into the water column.

Scheme of Classification:

Marshall and Williams (1964) pertain a single class Cephalochordate under the sub­phylum Cephalochordata, but without any order under the class Cephalochordate. Godeaux (1974) comprises a single order Amphioxi under the subphylum Cephalochordata but without any class.

Berrill (1950), Barrington (1979), Young (1981), Romer and Parson (1986), Ruppert and Barnes (1994), Kent and Miller (1997), Anderson (1998), Pechenik (2000) and Kardong (2002) do not mention any class and order under Cephalochordata. We follow in this text book Young’s (1981) scheme, adopted from the book “The Life of Vertebrates (3rd ed.)”.

Systematic Resume:

The subphylum Cephalochordata compris­es a single family Brachiostomatidae with two genera Branchiostoma (Costa, 1834) and Asymmetron (Andrews, 1893). Other genera as Epigonichthys is now synonym with Asymmetron and Amphioxides, once considered as member of the family Amphioxididae, now considered as giant larval individuals of the genus Asymmetron.

Genus 1. Branchiostoma [Approx. 16 species]

Gonads lie on each side of the body. They inhabit the tropical and sub-tropical seas.

Example:

B. lanceolatum (Sri Lanka, India, Mediterranean, N. W. Europe, eastern part of U.S.A.); B. belcheri (Sri Lanka, India, Torres Strait, Singapore, Borneo); B. capense (S. Africa); B. indicum (India, Sri Lanka); B. pelagicum (India); B. elongatum (Peru); B. nakagawae (Japan); B. caribbaeum (N. and S. America and West Indies); B. californiense (California, U.S.A.); B. tattersali (India).

Genus 2. Asymmetron [Approx. 7 species]

Gonads lie only on the right side. They inhabit the tropical seas.

Example:

A. cingalense (Sri Lanka); A. cultellum (Sri Lanka, Australia); A. lucayanum (Maldives, Bahamas, Zanzibar); A. caudatum (Louisiade Archipelago); A. bassanum (Austra­lia); A. maldivense (Maldives, Zanzibar); A. hectori (New Zealand).

Feeding Mechanism in Protochordates:

Most of the animals move from one place to another in search of food. But in partial or complete sessile animals, this particular function of locomotion is compensated by the develop­ment of special structures.

Majority of the inver­tebrate chordates are either sedentary or sessile and they have developed specialised devices to strain off micro-organisms from water. Isolation of food materials from water is done by the ciliary and glandular tracts. So the method of feeding is called ciliary mode of feeding.

The pharynx is modified in the invertebrate chor­dates to perform dual functions:

(1) Respiration and

(2) Food collection.

In all of them, a constant flow of water current passes into the pharyngeal cavity and goes out through the atriopore.

Structural elements associated with feeding:

The pharynx in the invertebrate chordates plays the most spectacular role in straining off the food materials from the incoming water current. For this physiologi­cal function, the pharynx and its associated structures become greatly altered.

The pha­rynx includes the portion of the gut from the posterior part of the mouth cavity to the beginning of oesophagus. Although the pha­ryngeal apparatus is basically similar in all these forms, some minor differences exist which depend on the degree of sessile habits.

Mechanism of food concentration:

The food of invertebrate chordates comprises of micro-organisms which are suspended in sea- water in a very dilute condition. The food particles need to be filtered out by removing the excess of water from the pharyngeal cavity. This phenomenon is effectively done by an efficient mechanism of food concen­tration.

The cilia of gill-bars in the pharynx beat synchronously and by their powerful move­ment a current of water is produced which, after entering the mouth, passes towards the pharynx. The velar tentacles in urochordates resist the entry of big particles which are un-digestible.

The same function is performed by the buccal cirri in the cephalochordates. In cephalochordates the wheel-organ creates a vortex of water and focuses it towards the mouth.

Before entering the pharyngeal cavity the velar tentacles sieve-off sand grains and other unwanted particles. Enormous quantity of water containing food enters into the pha­ryngeal cavity and most of the water goes out to the atrium in urochordates and cephalo­chordates and directly to the outside in hemi­chordates through the gill-slits or their deriva­tives.

In hemichordates, the food particles along with the sand grains are entangled by the mucus secreted from the glands present in the mouth cavity and pass through the pharynx into the intestinal cavity. The mucus secreted by the proboscis entangles the sand and other food particles.

The mucus-coated food particles are pushed directly into the mouth by ciliary action. This transit is assisted by the ciliary beatings of the ciliated strips so that the food matters are driven towards the intestine. Digestion occurs in the intestine and the sand grains are eliminated in the form of castings.

In urochordates and cephalochordates the endostyle entangles the food by its viscous secretion of mucus. The cilia of the endostyle transform the food into a mucous rope which is driven forward towards the peripharyngeal groove wherefrom it travels backwards through the hyper pharyngeal groove in cephalochordates and the dorsal lamina in case of urochordates into the intestine by cil­iary action.

In the intestine the food is digest­ed and the undigested products are discharged through the anus.

The most important feature of the ciliary feeding in invertebrate chordates is that selec­tion and filtration of food are done by physical means. This method of feeding is a variant of the general process observed in other forms where the sorting of the food particles is done inside the pharyngeal cavity.

The presence of numerous gill-slits is an adaptive feature which facilitates the elimination of excess of water from the pharyngeal cavity and also helps in the process of gaseous exchange.

The pharynx of the invertebrate chordates has been specialised and elaborated as a food-concentrating apparatus in addition to its nor­mal respiratory function. This apparatus shows close similarity in different invertebrate chor­dates.

The degree of specialisation of the pha­rynx in these forms bears a distinct ratio to their sedentary habits. The close similarities in the structure and function of the pharyngeal apparatus are suggestive of the original habit of the earliest ancestral chordates from which they have evolved.