Notes on Mesozoans (With Diagram)

The following points highlight the Two main types of phyla present in Mesozoans. The types are: 1. Phylum Orthonectida 2. Phylum Rhombozoa.

Mesozoans: Phyla Type # 1. Phylum Orthonectida:

Features:

1. Microscopic and worm-like animals, and the body length rarely exceeds 300 µm.

2. Body without organs and consists of an outer layer of multi-ciliated cells which enclose an inner mass of eggs and sperm cells.

3. In some species the circular and lon­gitudinal muscles are present beneath the epidermis.

4. Parenchymal cells absent but some ex­tracellular matrix present.

5. Locomotion by cilia.

6. Fertilization takes place outside the host.

7. Most species show gonochoristic sexu­ality.

8. Within host tissue the larvae losing their ciliated cells become the form of asymmetrical, multinucleate but syn­cytial amoeba-like individuals, called Plasmodia (Fig. 14.30B). A Plasmo­dium produces both male and female ciliated adults (Fig. 14.30A).

Habitat:

They become parasitized on an­other marine invertebrates such as turbellarian flatworms, polychaete annelids, nemerteans, bivalve molluscs and echinoderms.

Example: Rhopalura (Fig. 14.30A, B).

Mesozoans: Phyla Type # 2. Phylum Rhombozoa:

Features:

1. Adult dicyemid is called nematogen and about 0.5 to 7 mm long.

2. Body composed of less than 30 cells.

3. Outer body layer composed of 20-30 ciliated cells in number that enclose single to several elongated axial cells which form a central shaft of the animal.

4. Anterior cells help to be attached with the host cells.

5. Within cephalopods the nematogen de­velops into a sexual stage, called rhombogen.

6. Self-fertilization common.

7. After fertilization ciliated infusoriform larva develops from the zygote.

8. Life cycle is not completely known.

Habitat:

They are found in the nephridia of cephalopod molluscs such as cuttle fish and octopuses, and obtain the nourishment from host urine in which they live.

Phylogenetic Relationship:

The phylogenetic position of the mesozoans is not fully clear. The structural organizations indicate that mesozoans may be between the protozoans and triploblastic flatworms due to presence of two germ lay­ers ectoderm and endoderm (Ruppert and Barnes, 1994).

But Pechenik (2000) suggests that mesozoans represent an early branch of metazoan evolution due to the intracellular development of the reproductive cells and embryos.

Example:

Dicyemmenea, Microcyema.