An Example of Phylum Rotifera: Brachionus

In this article we will discuss about Brachionus:- 1. Habit and Habitat of Brachionus 2. External Structures of Brachionus 3. Body Wall and Body Cavity 4. Digestive System 5. Circulatory System 6. Excretory System 7. Nervous System 8. Reproductive System 9. Development.

Contents:

1. Habit and Habitat of Brachionus
2. External Structures of Brachionus
3. Body Wall and Body Cavity of Brachionus
4. Digestive System of Brachionus
5. Circulatory System of Brachionus
6. Excretory System of Brachionus
7. Nervous System of Brachionus
8. Reproductive System of Brachionus
9. Development of Brachionus

1. Habit and Habitat of Brachionus:

Brachionus is a fresh-water inhabitant, and feeds on microscopic organisms brought by ciliary action.

2. External Structures of Brachionus:

Brachionus exhibits distinct sexual dimor­phism. The females are well-formed and are larger in size in comparison to their male counterparts (Fig. 15.18 A, B). The male is different from female in having a number of peculiar features.

It is a minute degenerate form where the alimentary canal is lacking. All other organs, excepting the reproductive organs, are greatly reduced. The trochal disc is simple in males. The following description implies to a female Brachionus.

The body is divisible into trunk and tail or foot. The trunk is encased in a transparent flexible lorica (Fig. 15.18A) which is pro­vided with spines. The tail is wrinkled and terminates into two small processes. Mouth is situated on the ventral side of the trunk. The dorsal side is convex. The anterior end of the body projects out as trochal disc which is a transverse disc-like body beset with cilia in the margin.

The trochal disc helps in loco­motion by rotation. A pair of cement glands are present in the tail. The secretion of the cement glands enables the animal to attach in the substratum temporarily. It has a re­markable power to survive in desiccation.

3. Body Wall and Body Cavity of Brachionus:

The body wall is composed of a cuticle, epidermis and sub-epidermal muscles. The epi­dermis is syncitial and contains very few gland cells. The body cavity is a pseudocoel.

4. Digestive System of Brachionus:

The food of Brachionus includes minute organisms (specially protozoans) and organic debris which are brought into the mouth by ciliary action. The mouth is situated in a depression on one side of the trochal disc. The mouth leads into a very short buccal cavity. The buccal cavity opens into a round muscular chamber called the pharynx and mastax.

The mastax is a complicated struc­ture which varies in shape and size. Several types of mastax are encountered in rotifers, but the particular type of mastax present in Brachionus is of malleate type. The mastax is a peculiar apparatus having characteristic of the rotifers. It is composed of masticatory (trophi) and musculature.

Typically the trophi consists of two chief components: (A) Incus and (B) Malleus. The incus is again made up of a single basal piece called fulcrum and two lateral triangular pieces called rami.

The malleus is composed of two toothed pieces of unci and two pieces of elongated hammer-like manubria. These different components are operated by sets of well-developed muscles. About 2-7 salivary glands are situated in the mastax wall and the ducts, specially of the ventral pairs, open anterior to the trophi.

Due to different feeding adaptations, the mastax becomes modified in different spe­cies. The varieties of mastax found in differ­ent rotifers have great taxonomic signifi­cance, because they are regarded as the cri­teria for classification.

The pharynx of Brachionus is connected with spacious thick-walled sac-like stomach by a short oesophagus. A pair of digestive glands are associated with the anterior end of the stomach.

From the stomach a short intestine emerges which passes into an oval cloaca. The cloaca communicates to the exte­rior through the cloacal aperture. This aper­ture is located mid-dorsally towards the posterior end. The inner wall of the stomach and intestine is ciliated.

5. Circulatory System of Brachionus:

Brachionus lacks a definite circulatory system.

6. Excretory System of Brachionus:

The excretory system comprises of a pair of nephridial tubes or excretory canals which open posteriorly into a contractile vesicle or urinary bladder. The excretory canals are situ­ated one on each side of the alimentary canal. The contractile vesicle opens into the cloaca. The cavity of the nephridial tubes is intracellular in nature.

7. Nervous System of Brachionus:

The nervous system consists of large bilobed ganglion (Brain) situated on the dorsal side of the body above the pharynx. From the brain, many nerves emerge out to supply the various organs of the body including the sense organs.

Sense Organs of Brachionus:

Chief sense organs are present in the forms of very small eye-spots or ocelli on the dorsal aspect of the brain. The other sense organs are one dorsal and two lateral tactile rods or feelers.

8. Reproductive System of Brachionus:

The sexes are separate and the sexual dimorphism is well-marked. The female is comparatively larger than the male. The fe­male reproductive system consists of an ovary which is communicated to the cloaca by an oviduct. The ovary is also designated as the germarium and is connected with a large vitellarium which is a syncytial tissue and produces yolk filled nurse cells. In Brachionus, three types of eggs are produced.

These are:

(a) Thin-shelled amictic eggs which cannot be fertilized and develop by parthe­nogenesis into amictic females. These females lay diploid eggs for most of the year.

(b) Thin- shelled haploid mictic eggs.

Unfertilised mictic eggs produce males by parthenogen­esis and fertilised mictic eggs are called resting or dormant mictic eggs and some­times called winter eggs.

The dormant mictic eggs after fertilization produce a heavy impervious resistant shell and hatch into amictic females. Sometimes female produ­cing amictic eggs and male producing mictic eggs which develop parthenogenetically called summer eggs.

Amictic and mictic females do not differ anatomically but they differ physiologically. The male reproduc­tive system consists of a very large testis which leads into a duct opening into a dorsally placed penis.

9. Development of Brachionus:

The zygote divides into micromeres and macromeres by unequal holoblastic cleav­age. Gastrulation occurs in epibolic fashion. The embryo transforms directly into adult.

(i) Corona:

The cephalic region of the body bears a wheel-like ciliated organ in rotifers called the corona or rotatory apparatus which is the characteristic of all members of this phylum.

In primitive type (Fig. 15.19B) a ciliated band encloses the apical area and is ex­tended posteriorly to the ventral side. In this type the corona consists of a large ventral ciliated area called buccal field which sur­rounds the area of mouth. From the buccal field cilia are extended around the anterior margin of the head to form a crown-like ring called the circumapical band.

The inner area of the ring which is devoid of cilia called the apical field. This type is found in many forms of Notommatidae (e.g., Scaridium) and is considered as the basic structural plan and modifications include alterations of cilia into cirri, membranelles or bristles.

In advanced form the original buccal field and circumapical band are either well- developed or lost. The cilia of circumapical band are modified to form a large tuft called auricle and on the apical field are modified as cirri. The corona in Abrochta includes an unpaired ciliated band.

In Adineta, a homogenous ciliated field is on the ventral part of head which hangs like a lip. In some free- swimming forms (e.g., Brachionus, Euchlanis) the cilia of the margin of the buccal field are enlarged and modified into stiff cirri or a series of tufts.

In some species (e.g., Euchlanis) the elongated cilia are restricted to the lateral projection of the body wall called the auricle.

In Lacinularia, Philodina and Rotaria, there are two ciliated rings, of which the inner elon­gated ciliated ring anterior to the mouth is called trochus and posterior (outer) elon­gated ciliated ring that passes round the mouth is called cingulum. In bdelloids, the head is projected into rostrum which bears cilia and sensory bristles at its tip.

In Collotheca the corona is modified to form a funnel surrounding the mouth. The margin of this infundibular corona is lobed which is pro­vided with long bristles. In some species of Hexarthra and Testudinella the corona is re­duced as the buccal field is short and circum apical band is totally absent. The coronal cilia help in swimming and are also used in capturing food.

(ii) Mastax:

The muscular pharynx containing inter­connected movable chitinous pieces acting as jaws is called mastax. The presence of mastax is the characteristic and peculiar to the rotifers. It is the diagnostic feature in taxonomy and the structure of mastax helps to draw the affinities of rotifers with other groups.

Gosse (1852) first gathered some information about the masticatory apparatus of rotifers regarding its structure, function and homology. More comprehensive account was advanced by Hudson and Gosse in their monumental monograph in 1886.

Structure of a typical mastax:

The mastax is a highly muscular, compli­cated structure which varies in shape and size. The inner walls carry large, hard, cuticularised interconnected pieces called jaws or trophi or dental mill used to grasp, cut and grind the food following ingestion. The mastax of some parasitic species is modified for the attachment to the host’s body. The trophi of some species is used to suck food in through the mouth.

The trophi or masti­catory apparatus includes seven cuticularised plates, composed of an unpaired ful­crum and paired rami, unci and manubria. The fulcrum and rami together are called incus, and unci and manubria together called malleus. In the midline there is a forked structure called anvil or incus which is made up of a single, midventral basal piece called fulcrum.

The fulcrum bears two lateral trian­gular pieces called rami. On either side of incus there is a hammer-shaped malleus which consists of an anterior toothed piece or uncus and elongated handle-like manu­brium embedded in muscles. The posterior end of the manubrium is known as cauda.

Muscles:

The different components of the mastax are operated by sets of well-developed mus­cles. These muscles extend between pieces of the trophi and from the trophi to adjacent soft parts.

These include:

1. Abductor-rami (Fulcro scapalis):

This muscle helps to open the rami.

2. Adductor rami:

Runs between the rami and closes them.

3. Flexor-mallei:

The muscle extends from cauda to rami and helps the unci to strike against the rami.

4. Uncicus:

This muscle attaches to the uncus and both ends of manubrium. This muscle is associated with flexor mallei which act to rub the uncal teeth on the ramus.

5. Adductor mallei:

The chief muscle which produces the chewing movement of the unci.

Some muscles which extend from the trophi to the mastax walls or regions outside the mastax, are fulcro-oralis, fulcro-mucosus, fulcro-oesophagus and scapalis.

Salivary glands:

About 2-7 salivary glands are situated in the mastax wall and the ducts, specially of the ventral pairs, open anterior to trophi or near the mouth or in the buccal tube. The salivary glands in many rotifers are uninucleate or syncytial masses with granular or vacuolated cytoplasm. The function of the salivary glands is probably concerned with digestion.

Types of mastax:

Due to different feeding adaptations, the structure of trophi becomes modified in dif­ferent species. The varieties of mastax found in different rotifers have great taxonomic significance.

The types of mastax are:

(a) Malleate type:

This type represents the most primitive condition and all the components in it are relatively short and strongly built. The rami are de­void of teeth but the unci bear a few prong-like teeth (Fig. 15.18C). It assists in grasping as well as chewing the prey. This type is found in Brachionus, Epiphanes.

(b) Sub-malleate type:

Mallei are slender and manubria twice as long as unci. This type is found in Euchlanis.

(c) Virgate type:

The rami are thin, and fulcrum and manubria are elongated and rod-like. The unci contain only one or two teeth (Fig. 15.20A). The unci are used to grasp the prey and the contents of the prey are sucked by hypopharynx. This type is found in pelagic raptorial rotifers (Scaridium, Trichocera, Polyarthra, Synchaeta).

(d) Cardate type:

This type is involved in sucking and the suction is produced by the movements of the unci. An epipharynx of several pieces is present and the manubria are forked. It is also modified for pumping. This type is found in Lindea.

(e) Forcipate type:

All the pieces of the trophi are elongated and slender. The rami are curved and dentate. The tips of the rami are sharply pointed and intimately associated with the rod-like pointed unci. The rami and the ful­crum together form a forceps-like structure. The forcipate-type of mastax can protrude through the mouth to grasp the prey. This type is found in Dicranophorus (Fig. 15.18D).

(f) Incudate type:

This type is similar to forcipate type but the incus are en­larged, rami toothed and mallei are vestigial (Fig. 15.20B). This type of mastax helps to grasp the prey. It is found in Asplanchna.

(g) Ramate type:

This type is the character­istic of bdelloids. Fulcrum and manu­bria are reduced and unci are large and plate-like and provided with sev­eral ridges (Fig. 15.20C). The trophi serve to grind the food.

(h) Malleoramate type:

This type of mastax is seen in Lacinularia, Testudinella. The crescent-shaped manubrium borders the edge of uncus. Rami are long and narrow and unci are multi-toothed. This trophi serve to grind the prey. The mastax of suspension feeders is used to grind the food.

(i) Uncinate type:

This type of trophi are provided with teeth. Sometimes only one tooth with several accessory teeth are on the unci. Rami are stout and large subunci. The trophi are used for laceration and seen in Collotheca.

(j) Fulcrate type:

This type of mastax is seen in Seison. In the fulcrate type the ful­crum is elongated and manubria are attached to the anterior of the fulcrum. The Seison is a parasitic species and the mastax is used for the attachment to the gills of polychaete.