Diploblastic and Triploblastic Organisation, Coelom, Segmentation and Notochord

Diploblastic and triploblastic organisation. During embryonic development, the cells of the embryo arrange themselves into distinct layers, called germ layers, from which all the adult tissues and organs are eventually built. Animals whose adult body is derived from only two such germ layers — an outer ectoderm and an inner endoderm, with an undifferentiated, often jelly-like layer called mesoglea lying between them — are called diploblastic; the coelenterates and ctenophores are diploblastic. Animals whose adult body is derived from three germ layers — ectoderm, mesoderm and endoderm, with the middle mesoderm layer giving rise to most of the skeletal and muscular tissue — are called triploblastic; every phylum from the flatworms onward, including all the chordates, is triploblastic, and the presence of a true mesoderm is what makes complex organs and organ systems possible in the first place.

Coelom. The mesoderm of a triploblastic animal may or may not enclose a fluid-filled body cavity lying between the gut and the body wall, and the presence, absence or nature of this cavity is itself a major basis of classification. Where the mesoderm forms a body cavity that is completely lined by mesoderm on all sides (both the wall touching the gut and the wall touching the body-wall musculature), the cavity is a true coelom and the animal is described as coelomate — annelids, arthropods, molluscs, echinoderms and chordates are all coelomate, though the mode of origin of the coelom differs between groups: schizocoelous, arising from a split within a solid mass of mesoderm, in annelids/arthropods/molluscs; enterocoelous, arising by an outpouching of the embryonic gut wall, in echinoderms and chordates. Where a body cavity is present but is not fully lined by mesoderm — mesoderm occurs only as scattered patches or is present only around the gut, so the cavity remains partly bounded by endoderm — the cavity is called a pseudocoelom and the animal is pseudocoelomate; the aschelminthes (roundworms) are the standard example. Where there is no body cavity at all between the gut and the body wall, the space instead being packed with a loose parenchyma tissue, the animal is described as acoelomate; the flatworms are acoelomate.

Segmentation. In some phyla, the body is externally and often internally divided into a linear series of segments, each segment repeating (at least in the pattern-forming stage) many of the same organs — a condition called metameric segmentation, or metamerism. This first appears in the annelids, whose very name (from the Latin annulus, a ring) reflects it, and it continues, in a more specialised form, in the arthropods, whose segments are often fused into functional body regions such as a head, thorax and abdomen. …