Rotifera (description and main features of Rotifera)

ROTIFERA 

DESCRIPTION

Rotifera are found in abundance in freshwater. These are also known as wheel animalcule. These are the most minute animals of Metazoa. The body of these animals is transparent and inner body activities can be observed form outside in living state. 

These animals were discovered by Leeuwenhoek (1703). Initially, they were placed in protozoa groups. Ehrenberg, (1838) placed them separately in class infusoria. The word Rotifera has been originated from two Latin word rota(meaning: wheel) and ferre(meaning: to bear). Hence, Rotifera means wheel bearers. 

These animals were named because the cilia present on the disc-shaped lobes on their head move in sequence giving the impression that a wheel is moving on their head. 

Main features of Rotifera are-

(1) They are microscopic animals. 

(2) Body is unsegmented. 

(3) Bilateral symmetry is found. 

(4) These are free-floating animals in which a ciliary apparatus known as corona, is present at the anterior end which helps in locomotion and feeding. 

(5) There is a muscular pharynx which has movable jaws. 

(6) Excretion occurs through flame bulb nephridia which directly open in cloaca. 

(7) There is no any separate organ for respiration and fluid circulation is found. 

(8) Male and female sexes are separate and Structurally different. In this way they are dioecious and there is sexual dimorphism in them. 

Digestive system of Balanoglossus (Phylum- Hemichordates)

(i) Alimentary canal:

In alimentary canal of Balanoglossus is a straight pipe-like structure situated antero-posteriorly in the body. It is divided into four parts--

(1) Mouth:

It is a broad circular pore situated in between proboscis stalk and collarette on the ventral surface. It is permanently opened.

(2) Buccal tube and Buccal cavity:

The short buccal cavity lies in the collar region. It is lined with ciliated and glandular cells. From its roof arises a stiff, hollow buccal diverticulum extending into the proboscis coelom. Posteriorly it extends up to the collar-trunk septum and then extends into the trunk as pharynx.

(3) Pharynx:

It is situated in the bronchial region of the trunk. One both of the lateral sides of its wall lateral longitudinal constrictions are found. Each of these lateral longitudinal constriction protrudes one in the pharynx and are known as parabranchial ridges. These are lined by long columnar cells. These lateral longitudinal constrictions and parabranchial ridge divide the pharynx in an upper clome or branchial part and the second lower digestive part.

(i) Branchial part:

It is an upper part of pharynx related with respiration. A row of gill pores is situated on each of its dorsolateral surfaces. Each gill pore is U shaped and opens in a branchial sac. This has been well discribed in respiration process.

(ii) Digestive part:

It is lower part of pharynx which is concerned with digestion of food and its absorption. Its inner wall is lined by ciliated epithelial cells which contain glandular cells also.

(iii) Oesophagus:

Behind the last pair of gill-slits the pharynx continues into the oesophagus. The dorsal and ventral division of the pharynx continues for some distance into the oesophagus; in this region, the dorsal part of the oesophagus is called post-branchial canal which possesses thick, folded and glandular epithelium. The posterior part of the oesophagus reduces in diameter and has deeply furrowed epithelium.

(4) Intestine:

Behind the oesophagus is an intestine. It occupies the hepatic and post-hepatic regions of the trunk. The hepatic region of the intestine is highly vascular. Its epithelial cells are dark green or dark brown and its dorsal wall forms numerous prominent sacculations called hepatic caeca which push the body wall outwards and are, thus, visible externally.

The post- hepatic region of the intestine is connected with the ventral body wall by the pygochord. The intestine has the form of a simple tube and bears a pair of dorso-lateral grooves lined by tall epithelial cells bearing long cilia.

(5) Anus:

Posteriorly the intestine opens out through the anus situated at the extreme hind end of the body. The anus often possesses sphincter muscles.

(ii) Food and feeding mechanisms:

Balanoglossus is a ciliary feeder animal. Its main food are microscopic organisms and inorganic particles which are found on bed sand and in water around its burrow. Lateral cilia lining the gill pores produce water current which enters into mouth cavity through mouth opening and moves through pharynx, gill slits and gill sacs to come outward through gill pores. This water current is known as respiratory cum food water current. It helps both in food absorption and respiration.

Some food particles coming through the water current directly enter into mouth whereas some get entangled into mucous membrane after coming in contact with proboscis. This mucous is secreted by glandular cells present in proboscis epithelium. These cilia lining the proboscis, pass the food particles containing mucous membrane towards the pre-oral ciliary organ at the base of proboscis in the form a mucous chord. The pre-oral ciliary organ now pushes particles on mucous choro towards posterior part of mouth.

There are U shaped epidermal depressions at the base of the ventral layer of proboscis. These are surrounded by long epidermal cells containing long cilia. This Structure is known as pre-oral ciliary organ. These organ examine the food particles and water coming in the mouth. After testing the food particles not useful for entering into mouth are stopped to enter into mouth by lower part of collarette. This action is accomplished by shutting the mouth and the unwanted food particles do not enter and pass posteriorly through the upper collar region

(iii) Digestion:

Cilia present in the alimentary canal continously push the food backward inside this canal. Food moves through lower enzymatic region of pharynx where some enzymatic glandular cells present there secrete and mix the enzymes in the food. Such enzymes help in the digestion of the food. There is absorption of digested food in the intestine. Rest of the sand and silt particle coming through the water current are expelled outside with undigested food through the anus. 

Short information about ASTERIAS (star fish)

INTRODUCTION AND DEFINATION.

Radially symmetrical metazoans which have calcified ossicles and spicules in their leathery skin (Gr. Echinos = spiny, dermates = skin). Hence they are known as spiny skinned animals. Their 6750 living species and 20,000 extinct (fossilized) species have been reported so far. All are exclusively marine.

They have as endoskleton composed of calcareous paltes and spines. Some very beautiful animals like star fishes, sea-cucumbers, brittle stars, sea-urchins and sea-illies are included in this phylum.

Asterias which is familiar to everyone and which is commonly called as star fish comes under class Asteroides of this phylum. Its specific features is central disc from which five (or more) arms originate. These are not actual fishes therefore it will be more appropriate to call them sea star instead of star fish.

Luidia, Pentaceros, Solater, Heliaster and Asterias etc. Are common genera of sea stars.

Classification

Kingdom = Animalia 

Phylum = Ecinodermata 

Class = Asteroidea 

Order = Forcipulatida 

Family = Asteriidae 

Genus = Asterias.

EXTERNAL FEATURES

(1) Shape and size - sea stars have pentamerous radial symmetry which is originated from by the bilateral symmetry of its larva. These are star-shaped and are about 25cm in diameter. There is an indistinct central disc in the body of ASTERIAS from which arise five long and tapering arms. In few animals of this class the number of arms are even more than five; for example in SOLASTER there are 7 to 14 arms.

(2) Colour - It is pink, orange, jambolin-like or yellowish in colour. It's dorsal or abnormal surface is of dark colour.

(3) Structure - The body is flattened and is distinctly divided into oral and aboral surface.

ORAL SURFACE AND ABORAL SURFACE OF STAR FISH.

The madreporite is located on the aboral surface of the sea star, which is the side opposite the mouth. Most people would consider this the "top" of the sea star.

EXPLANATION. 

Sea stars have two surfaces: the oral and the aboral. The oral is the side with the sea star's mouth and most people would think of it as the "bottom." The aboral is the opposite side, and this is where the madreporite lies.

The madreporite is the opening to the water vascular system. It takes in water that passes through the system and allows the sea star to move using its tube feet.