In this article we will discuss about the development characteristics of protostomes and deuterostomes.
Protostomes:
Developmental Characteristics:
1. Pattern of embryonic cleavage:
Spiral cleavage pattern; the spiral cleavage is masked at the 6th cleavages (64-cell stage), e.g., Sipuncula, Echiura, Annelida, Pogonophora, Mollusca and some groups of Arthropoda (e.g., Myriapoda, Crustacea and Insecta except Chelicerata).
2. Fate of embryonic blastomeres:
Determinate or mosaic pattern development.
3. Fate of blastopore:
The blastopore is either becomes mouth (e.g., Mollusca) or gives rise to both mouth and anus (e.g., some molluscs, polychaetes and onychophores) in adult.
4. Formation of mesoderm:
From the 4th cell which is also called mesentoblast and increases its number by proliferation.
5. Formation of coelom:
Coelom originates by the splitting of a solid mass of mesodermal band (schizocoely).
6. Arrangent of coelomic cavities:
The coelomic cavities are variable in number in different groups. In sipunculans and echiurans the body cavities are two in number, but in echiurans a septum separates the body cavity into two. In annelids the single coelom is divided by many septa.
7. Larval type:
The most characteristic larval type is trochophore.
8. Larval ciliary bands:
Compound cilia from multi-ciliated cells.
Examples:
Coelomate protostomes (e.g., Sipuncula, Echiura, Annelida, Pogonophora, Mollusca, Onychophora, Tardigrada, Pentastomida and some groups of arthropods).
Deuterostomes:
Developmental Characteristics:
1. Pattern of embryonic cleavage:
Radial pattern of embryonic cleavage (e.g., phoronids, echinoderms, chaetognaths, hemichordates and chordates excepting sea squirts (bilateral cleavage).
2. Fate of embryonic blastomeres:
Indeterminate and regulative development.
3. Fate of blastopore:
Blastopore becomes the adult anus and then the formation of mouth takes place from a second opening on the dorsal surface of the embryo.
4. Formation of mesoderm:
From the sides of archenteron as a hollow coelomic pouches.
5. Formation of coelom:
Evagination of pouches from the wall of archenteron and each diverticulum becomes separated from the archenteron and develops independent coelomic pouch (enterocoely).
6. Arrangement of coelomic cavities:
A Tripartite coelom.
7. Larval type:
Dipleurula.
8. Larval ciliary bands:
Simple cilia; single cilium in each cell.
Examples:
Echinodermata, Hemichordata and Chordata. Some developmental pattern is seen among lophophorates (e.g., Phoronida, Brachiopoda and Ectoprocta) and their inclusions within deuterostomes are in controversy.
Phylogeny of Coelomate Protostomes:
The coelomate protostomes include two lineages—one group includes echiurans, annelids, pogonophorans, onychophorans and arthropods which are characterised by segmentation and spiral cleavage, and another group which is non-segmented includes sipunculans and molluscs which have evolved from non-segmented ancestor.
The absence of segmentation in echiurans, sipunculans and molluscs is a secondary loss (Ruppert and Barnes, 1994).
Based on current molecular data, it has been suggested that the coelomate protostome animals can be divided into 2 groups:
1. Ecdysozoa, e.g., Arthropods and Nematodes (pseudocoelomates)
2. Lophotrochozoa, e.g., Molluscs and Annelids.
Phylogeny of Deuterostomes:
The deuterostomes include Echinodermata, Hemichordata and Chordata. With these groups, the other few minor groups such as lophophorates and chaetognaths are included under deuterostomes but in considerable controversy.
The echinoderms, hemichordates and chordates share some common features such as gill slits (absent in living echinoderms but are found in fossil carpoids), protocoelic nephridium (present in echinoderms and hemichordates but in chordates it has secondarily lost).
All the five mentioned groups are included under deuterostomes on the basis of embryonic development features. Recent molecular data such as 18SrRNA and mitochondrial DNA gene sequences of lophophorate phyla indicate that they (e.g., Phoronids, Brachiopods and Ectoprocts) place in the protostomes.
Barnes (1987) has suggested that chordates evolved from non-chordata group and hypothetical echinoderm larva (dipleurula) and other echinoderm larvae hold the key position. According to him, the evidences of phylogenetic relationship between echinoderms hemichordates and chordates are very convincing on the basis of larval stages.
Hyman (1940) has stated that all multicellular animals have evolved from some single-called protest, probably from a colonial flagellate and some advanced phyla which are grouped as protostomes and deuterostomes, originate separately through the trochophore and dipleurula larva.
