Trochophore Larva Trochophore Gr Trochos wheel phoros bear

Trochophore Larva Trochophore (Gr. Trochos= wheel; phoros= bear) The cilia of ciliary girdles, rings or bands, when beating give the appearance of a rotating wheel

TROCHOPHORE LARVA • The stage Trochophore larva is present in many protostomous groups (e. g. , molluscs) • It is minute, ciliated, unsegmented, pear shaped and pelagic, with oral and aboral surfaces • It exhibits two stages in its life 1. the early, pre segmental trochophore 2. segmented trochophore larva

The early, presegmental trochophore has : 1. Prototroch : a locomotor ciliary band anterior to the mouth region 2. An apical ciliary tuft associated with the apical sense organ 3. Telotroch (a perianal ciliary band called) 4. ocelli and statocyst 5. a pair of protonehpridia

Characteristic features of a fully formed Trochophore larva • Sensory apical organ bearing a tuft of cilia • Brain, beneath the apical organ • One preoral ciliary band (= prototroch) just above the equator of the body • One postoral ciliated band (= metatroch) behind the mouth • Another ciliary band, the telotroch, in front of the anus • All these ciliary bands help in locomotion as well as in feeding • Digestive tract of trochophore larva is complete • Fluid filled blastocoel surrounds the larval gut • Neither segmentation nor coelom

• Trochophores may be planktotrophs (i. e. , feeding on plankton) and have long planktonic lives, Or lecithotrophs (i. e. , a non feeding larva which utilizes yolk as a source of nutrition) and have a short planktonic existence

The trochophore may be divided into three regions: 1. pretrochal region surrounding the mouth 2. pygidium consisting of telotroch and anal area 3. growth zone, lying between the mouth and telotroch Note: In Polychaetes, the growth zone forms all the trunk segments

Growth and development • proliferation of tissue in the growth zone results in growth and elongation • After metamorphosis, few larval structures are lost, i. e. , protonephridia, muscle bands, ciliary girdles, apical plate and brain • Segmented forms of larvae are called polytroch larvae • The region anterior to the protrochal ring forms the prostomium, while the protrochal region develops into peristomium • Size increases gradually and the body continues to elongate (with the addition of new segments) • finally shifts from plankton life to the life of a young polychaete

Affinities with Rotifera • Trochosphaera, a rotifer, have simi larities with the trochophore larva of anne lid • ciliated girdles, posi tion of nervous system (‘Brain’), the sense organ, curvature of intestine, nephridia, position of anus • However, these resem blances are quite superficial and need further examination

Affinities with Veliger larva of Molluscs • Trochophore larva resembles Veliger larva in having the pre oral ciliated ring, ciliated tuft of flagella and apical plate • These similarities may be due to phylogenetic convergence

Significance of Trochophore larva • Animals like polychaetes, molluscs, develop a trochophore larva • The phyla like Platyhelminthes and Arthropoda, are also included in the above animal group, however they might have lost the trochophore larval stage during the course of their evolution • Resemblance between rotifers and the trochophore larvae of annelids and molluscs indicates that the rotifers are closely related to the ancestors of the annelids and molluscs • Trochophore theory was given by Hatscheck • Trochophore theory says that these animal might have evolved from a common ancestor called trochozoon (have trochophore like features) • Similarities of trochophore with other animals indicate that bilateral symmetry emerges from radial symmetry and trochophore larva represents a transitional stage