Main Features of Development In Invertebrates

Cleavage

It takes place before gastrulation. The two main types of cleavage is spiral cleavage and radial cleavage

Spiral Cleavage

Spiral cleavage is holoblastic when the egg contains little or no yolk (isolecithal egg); all the eggs divide at each cleavage.

Types of Cleavages

It is characterized by the axes of the cleavage spindles being oblique to the polar axis of the egg or blastula, rather than at right angles or parallel to it. Thus, the blastomeres become spirally arranged.' This type of cleavage is of protostomes i.e. seen in platyhelminthes (flatworms), nemerteans (Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms"), annelids, mollusks and arthropods. The blastomere even if separated as early as the first or second cleavage division, will only develop into that part of the animal that they would have formed if left in the entire egg. Thus, their fate is determined in some way very early in development so the spiral cleavage is called determinate cleavage.

Radial Cleavage

It is also holoblastic. The plane of cleavage is always either at right angle or parallel to the polar axis of the blastula. This type of cleavage is indeterminate, the fate of the various parts of the developing embryo being essentially plastic (moldable) until after gastrulation has occurred. This form of cleavage is seen in deuterostomes (Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "second mouth") are a superphylum of animals) consisting of echinoderms, hemichordates and chordates.

Gastrulation

In the case of determinate blastulae, the process is the moving into place the preordained parts while in the undetermined embryos it is the actual laying of the germ layers and in the initiating of determination for later development.

In the protostomes the mouth arises either from the blastopore itself or from a new opening very close to the original blastopore. In the deuterostomes on the other hand the mouth is completely new structure and the blastopore forms the anus.

In the spirally cleaved blastula, on completion of gastrulation the product of division of cells are the sites of invagination (Invagination is the enfolding of one part within another part of a structure, a folding that creates a pocket), grow into blastocoel to lay down the third germ layer, the mesoderm The various types of gastrulation is shown in figure . After gastrulation mesoderm formation rapidly takes place.

Development may be direct or through metamorphosis. Metamorphosis may be complete or incomplete in arthropods or through many larval stages in echinoderms.

Invertebrates are animals that lack a vertebral column, which includes a diverse range of organisms such as sponges, cnidarians, flatworms, mollusks, arthropods, and echinoderms. Although invertebrates vary greatly in their body structures, life cycles, and habitats, they share several features of development:

Gastrulation

During gastrulation, cells in the blastula rearrange themselves to form a two-layered or three-layered embryo, depending on the animal group. The formation of the germ layers sets up the basic body plan of the animal.

Various types of gastrulation

Differentiation

After gastrulation, cells within the germ layers differentiate into different cell types and tissues, giving rise to various organs and body structures.

Metamorphosis

Many invertebrates undergo metamorphosis, a process of dramatic change in body form and function as they transition from larval to adult stages. Metamorphosis can involve the loss of larval structures and the growth of new adult structures.

Indirect development

Some invertebrates undergo indirect development, in which the larval stage looks very different from the adult stage and may have different habitats and feeding behaviors.

Molting

Arthropods and some other invertebrates periodically shed their exoskeletons in a process called molting, allowing for growth and development.

The development of invertebrates is diverse and complex, reflecting the wide range of adaptations that have allowed these animals to thrive in a variety of environments.