INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION DEUTEROSTOMES (DEUTEROSTOMIA)

Deuterostomes are a major group of animal phyla sharing common structural and developmental features that set them apart from the invertebrate groups discussed previously.

Deuterostomes are characterized by a suite of traits, most of which relate to embryonic development. Cleavage is radial and predominantly indeterminate, resulting in a blastula that transforms via invagination into a two-layered gastrula. The blastopore of the gastrula does not become the Mouth, as it does in most invertebrates; instead, the anus forms at the blastopore site, while the mouth (the secondary mouth) develops on the ventral side of the opposite (anterior) end of the body (The phenomenon of deuterostomy, Fig. 420).

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Fig. 420. Features of deuterostome embryonic development (from Beklemishev):

a - gastrula; b, c - successive stages of secondary mouth formation in Balanoglossus (phylum Hemichordata); d - coelom formation in Saccoglossus kowalevskii:

1 - gut rudiment; 2 - ectoderm; 3 - blastopore; 4 - apical plate; 5 - coelom; 6 — secondary mouth;

7 - coelomic pore; 8 - anus; 9, 10 - coeloms II and III, respectively

The third germ layer (mesoderm) originates via lateral outpocketings of the entodermal gut, known as enterocoely. Initially, these appear as three pairs of lateral pouches (the first pair may be unpaired), which subsequently detach from the gut and position themselves on either side of it, forming coelomic sacs whose walls give rise to the adult mesoderm. Thus, the coelom in deuterostomes is tripartite, consisting of three pairs of coelomic sacs (the first pair may be fused), with the left ones being larger than the right ones. The tripartite Nature of the coelom is most pronounced in hemichordates; in Chordates, it is primarily the last, third pair of coeloms that develops; in Echinoderms, the coeloms undergo the most extensive reorganization, their tripartite Structure being evident only in larvae, whereas in adults, the coeloms give rise to several Organs.

A characteristic deuterostome larva is the dipleurula — a bilaterally symmetrical planktonic larva featuring a complete gut with a secondary mouth, three pairs of coelomic sacs, an apical plate with a Brain rudiment, and a sinuous ciliated band encircling the body. The dipleurula is typical of hemichordates and echinoderms; in chordates, it is absent due to the embryonization of development.

The Skin of deuterostomes consists of a single-layered ectodermal epidermis and a mesodermal cutis (dermis). The Skeleton, when present, is internal, cartilaginous or calcareous, and of mesodermal origin. The Nervous system typically appears as epithelial bands that in some cases sink beneath the epithelium to form nerve tubes, though a diffuse epithelial nerve net may also persist. The brain is secondary, when present, and forms as an expansion of the anterior end of the nerve tube rather than originating from the larval apical plate.

Deuterostomes comprise three phyla: Hemichordates (Hemichordata), Echinoderms (Echinodermata), and Chordates (Chordata). Modern molecular genetic studies strongly support the monophyletic unity of this group of phyla.