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. Above the prototroch is a sensory plate, an apical tuft of cilia, and an ocellus (simple eye). Below the prototroch are the mouth, stomach, anus, and other structures including the solenocyte, the function of which seems to be to maintain proper internal salt-water balance, and, in some species, one or two additional ciliary rings. In some mollusks (such as gastropods and bivalves), the trochophore develops into a second stage, the veliger (q.v.), before metamorphosing to adult form. Rotifers and the larvae (sometimes considered trochophores) of such invertebrates as phoronids and bryozoans are trochophore-like in appearance.

**Trochophore Larva in Annelida and Mollusca**

The trochophore larva is a type of free-swimming larval stage found in certain groups of animals. It is characterized by the presence of a ring of cilia, known as the trochophore, which is used for locomotion and feeding. The trochophore larva undergoes metamorphosis to develop into the adult form of the animal.

The correct answer to the given question is option 'D', which states that the trochophore larva occurs in Annelida and Mollusca. Let's understand why this answer is correct.

**Annelida:**
Annelida is a phylum of segmented worms that includes earthworms, leeches, and marine worms. Many annelids exhibit indirect development, where they go through a larval stage before becoming adults. The larval stage in annelids is known as the trochophore larva. The trochophore larva of annelids is characterized by a ring of cilia around the body, which is used for locomotion and feeding. This larval stage is found in various groups of annelids, including polychaetes and oligochaetes.

**Mollusca:**
Mollusca is a phylum that includes animals such as snails, clams, squids, and octopuses. Similar to annelids, many mollusks also exhibit indirect development and go through a larval stage known as the trochophore larva. In mollusks, the trochophore larva is also characterized by a ring of cilia that allows for movement and feeding. This larval stage is found in various groups of mollusks, including bivalves (such as clams and mussels) and gastropods (such as snails and slugs).

**Importance of Trochophore Larva:**
The trochophore larva is an important stage in the life cycle of these organisms. It allows for dispersal and colonization of new habitats as the larva can swim freely in the water. Additionally, the cilia on the trochophore larva aid in feeding, helping the larva to obtain nutrients from the environment. The larva eventually undergoes metamorphosis, transforming into the adult form of the animal.

In conclusion, the trochophore larva occurs in both Annelida and Mollusca. This larval stage is an important part of the life cycle of these organisms, allowing for dispersal and feeding before metamorphosis into the adult form.

In annilida and in mollusca (it's modified trocophore larva i.e. Veligers larva)....