Structural organization of leech - class-X

Class Hirudinea belongs to phylum Annelida. These are parasitic in nature which is equipped with suckers used for creeping. Leeches are primarily freshwater annelids, but some live in the ocean and some in moist soil or vegetation. The majority of leeches are predators on small invertebrates, most swallow their prey whole, but some suck the soft parts from their victims. Some leeches are parasites rather than predators and suck the body fluids of their victims without killing them.

Pseudopodia are the temporary extensions of the cytoplasm. These are used in locomotion and feeding. These are used by the protozoans and some of the flagellate protozoans. Polychaete worms  are the group of worms which belong to the phylum of Annelids. They are characterized by an elongated and segmented body. They bear a pair of appendages called parapodia on each metamere. These are used for locomotion. The organisms which are mollusc and belongs to the class of Gastropoda, have a flattened muscular foot for locomotion. They have a head that bears stalked eyes. Chondrichthyans are the group of cartilagenous fishes. Many chondrichthyans possess two dorsal fins, one anterior (just behind the head) and another posterior (in front of the tail).

Thus, the correct answer is option D. 

The scorpion heart is a strong, muscular organ extending along the dorsal mid-line of the heart.
Octopus has four pairs of arms, like all cephalopods.
Chamaeleon has pharyngeal gill slits in the embryo stage.
The central nervous system in leech is ventral.

Pontobdella is the type of leech. They are also known as skate sucker. It is the genus of leeches which includes bilaterally symmetrical animals. They belong to the class of Clitellata. It is marine leech. They are parasitic worms and attack skates and rays. They attach to the rock and wait to attack the skate. They also attack certain plaice. 

Archiannelids are a group of annelids that show internal segmentation by septa, but lack the external segmentation (annuli) characteristic of other annelid classes.

Hirudina (leeches) belong to the class Hirudinea, not Polychaeta. Polychaeta typically includes marine worms like Nereis.

Yellow cells (chloragogen cells) in oligochaetes and polychaetes are specialized cells found in the coelomic fluid that accumulate waste products, functioning similarly to a liver.

In polychaetes (or marine annelids) parapodia are present that help in gaseous exchange and in movement. Parapodia is absent in earthworm and in earthworms respiration is through skin. So, the correct answer is 'lack paddle-like parapodia that marine annelids have'.

Polychaetes are generally dioecious (separate sexes), whereas leeches, flatworms, and earthworms are typically hermaphroditic.

Lumbricus is a genus of earthworm, which belongs to the class Oligochaeta, not Polychaeta. Polychaetes are typically marine worms.

Pontobdella is segmented worm under the phylum Annelida. It is a marine leech which is ectoparasite on elasmobranchs and is commonly called as 'skate sucker'.
Therefore, the correct answer is option B.

Nephridia is an organ involved in the excretory system. It is the characteristic feature of leech. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. In leeches, the botryoidal tissue is composed of two different cell types. The granular botryoidal cells and flattened endothelial-like cells which contains a brown pigment and may serve an excretory function. Leeches use a combination of mucus and suction to stay attached and secrete an anticoagulant enzyme known as hirudin into the host's blood streams. 

Acanthobdella forms a connecting link between Oligochaeta and Hirudinea. It is mostly parasitic on fins of salmon fishes. Body comprises of thirty segments only. Its body cavity is spacious and completely divided by septa, vascular system is composed of dorsal and ventral vessel. Hence, option B is correct.

In leeches, the botryoidal tissue is composed of two different cell types-granular botryoidal cells and flattened endothelial-like cells containing a brown pigment and may serve an excretory function. Hence, option C is correct answer.

Hirudinaria is an Indian cattle leech. This is the fresh water species. It is cosmopolitan. It feeds on the blood of frog, animals and men. 

The tube-within-a-tube body plan refers to an organism having a digestive tract with two openings (mouth and anus), which is characteristic of annelids like earthworms.

Trochophore, also called trochosphere, small, translucent, free-swimming larva characteristic of marine annelids and most groups of mollusks. Trochophores are spherical or pear-shaped and are girdled by a ring of cilia (minute hairlike structures), the prototroch, that enables them to swim.  Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called setae, the project from the parapodia.