Trochophora

The structure of trochophores:
A - episifer
B - hyposphere
1 - ganglion
2 - sultan of cilia
3 - prototroch
4 - metatrach
5 - mesodermal germ
6 - anus
7 - nephridium
8 - intestines
9 - mouth
10 - blastocele

Trochofora (from the Greek. Τροχός - wheel and φορός - bearing) - free-floating larva of polychaete annelids (including echiurid and sipunkulid ) and mollusks , which has microscopic dimensions.

A characteristic feature of trochophores is the presence of ciliary cell belts (they are often called trochs in the zoological literature). Other planktonic larvae of marine invertebrates are similar to the trochophore: the Müller larva of ciliary worms, polyclade , nemertine pyridia , and phoronid actinotroch .

Trochophora is a typical planktonic larva floating with cilia . Her body has a more or less spherical or ovoid shape. At the front (animal) pole of the larva, a sensitive parietal sultan of long cilia develops, sitting on a group of ectoderm cells - the parietal plate. At the equator of the larva there is a characteristic preoral mouth corolla - prototroch. Sometimes a less powerful postpartum corolla is developed behind the mouth. The intestine begins with the mouth in the middle of the abdominal side of the larva and ends with powder at its posterior pole and consists of three sections, the anterior and posterior intestines being formed by ectoderma digestion , and the middle intestine is formed from the endoderm . Between the intestine and the wall of the body is the primary cavity of the body , intersected by thin muscle fibers. A pair of small protonephridia lies on the sides of the intestine. ^[one]

The mesodermal organs of the larva (mainly muscle fibers) develop from several cells lying at the edges of the blastopore and called mesenchyme . Another mezederm germ is represented by two large mesoderm cells - primary mesoblasts (sometimes called teloblasts). ^[one]

After a certain period of planktonic life, trochophores begin metamorphosis . The back (vegetative) hemisphere of the larva grows significantly in length and is divided into several segments at once. Parapodia and setae develop on segments. By this time, both primary mesoderm cells, multiplying intensely, give rise to two cellular strands lying on the sides of the intestine - mesoderm strips. Soon, under the influence of external segmentation, the mesoderm strips are divided into paired groups of cells, so that each segment has its own pair of mesoderm cell primordia. The latter are compact at first, then a cavity appears in them - the rudiment of the secondary cavity of the body, and the cell wall bounding it is the wall of the coelomic sac. Thus, in each segment, a pair of coelomic sacs develops. In the process of metamorphosis, a part of the cells of the parietal plate of the trochophore is immersed under the integument and forms the brain. On the ventral side in the form of a paired roller of ectoderm, abdominal nerve trunks are laid. In the future, they enter into communication with the brain with the help of near-pharyngeal connections. From the ectoderm, the sensory organs also develop - eyes, palps. ^[one]

The trochophora consists of three sections: the cephalic lobe, anal lobe and growth zone . In this area, a zone of future growth of the larva is formed. The trochophore plan at this stage resembles the organization of lower worms. The trochophore is subsequently transformed into a metatrochophore and a nektohet. In metatrochophores, larval segments form in the growth zone. Larval, or larval, segmentation captures only ectodermal derivatives: ciliary rings, protonephridia, the rudiments of the bristle sacs of future parapodia. Nektohet is different in that it forms the brain, the abdominal nerve chain. The bristles of the bristle bags are lined out, a parapordial complex is formed. However, the number of segments remains the same as that of the metatrochophora. Different numbers of polychaetes can have different numbers: 3, 7, 13. After a certain pause, post-larval segments begin to form and the juvenile stage of the worm forms. In contrast to larval segmentation, postlarval segments in juvenile forms capture derivatives of not only ectoderm, but also mesoderm . Moreover, in the growth zone, the teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a funnel of methanephridia is formed. The secondary body cavity gradually displaces the primary. At the boundaries of contact of coelomic sacs, dissipiments and mesentery are formed . Due to the remaining primary body cavity, longitudinal vessels of the circulatory system are formed in the lumen of mesentery, and ring vessels are formed in the lumens of the septum. Due to the mesoderm, the muscles of the skin-muscle sac and intestines, the lining of the coelom, gonads and coelomodules are formed. From the ectoderm, the nervous system, the channels of metanephridia, the anterior and posterior intestines are formed. Due to the endoderm, the middle intestine develops. After the completion of metamorphosis, an adult animal develops with a certain number of segments for each species of ^[2] worms.