Echinoderms and Chordates
Superphylum Deuterostomia contains two major groups you need to know: echinoderms (sea stars, sea urchins, sea cucumbers) and chordates (which includes vertebrates like us). What unites these seemingly different animals is a shared pattern of embryonic development, where the blastopore becomes the anus rather than the mouth. Understanding these developmental differences is one of the main ways biologists classify animal diversity.
Characteristics of Echinoderms
Echinoderms are exclusively marine. You won't find them in freshwater or on land. Their adult body plan shows pentaradial symmetry, meaning their body parts are arranged in fives or multiples of five (think of the five arms on a sea star).
Their most distinctive feature is the water vascular system, a hydraulic network of fluid-filled canals and tube feet used for locomotion, feeding, and gas exchange. Here's how it's organized:
- Madreporite: a porous plate on the body surface that lets seawater enter the system
- Ring canal: encircles the mouth and connects to the radial canals
- Radial canals: extend along each arm or ambulacral area
- Tube feet: muscular, fluid-filled extensions at the end of the system, used for movement, gripping prey, and feeding
Echinoderms also have an endoskeleton made of calcium carbonate plates called ossicles, which provide structural support. They display remarkable regenerative abilities and can regrow lost arms or other body parts.
One detail that's easy to overlook: echinoderm larvae are bilaterally symmetric. The pentaradial symmetry only appears after metamorphosis into the adult form. This bilateral larval stage is actually part of the evidence linking them to chordates as fellow deuterostomes.
Deuterostomes vs. Protostomes in Development
Three key developmental differences separate these two groups:
1. Blastopore fate
- In deuterostomes, the blastopore (the first opening that forms during gastrulation) becomes the anus. The mouth forms secondarily at the opposite end. The name literally means "second mouth" (deutero = second, stoma = mouth).
- In protostomes, the blastopore becomes the mouth, and the anus forms second.
2. Coelom formation
- Deuterostomes use enterocoely: the coelom forms when pouches bud off (outpocket) from the archenteron (primitive gut). The mesoderm originates from endoderm tissue.
- Protostomes use schizocoely: the mesoderm develops as a solid mass that then splits to create the coelomic cavity.
3. Cleavage pattern
- Deuterostomes show radial, indeterminate cleavage. Cells divide in parallel and perpendicular planes, and each early blastomere retains the ability to develop into a complete organism.
- Protostomes show spiral, determinate cleavage. Cells divide at oblique angles, and each blastomere's fate is fixed early on.
Distinguishing Traits of Chordates
All chordates share four defining features. Each one is present at some stage of development, though not necessarily in the adult.
Notochord: A flexible, rod-like structure running along the dorsal side of the body. It provides support and serves as an attachment point for muscles. In most vertebrates, it's replaced during development by the vertebral column (spine), but it defines the phylum.
Pharyngeal slits: A series of openings in the pharynx (throat region) that connect to the outside. In aquatic chordates, these function in filter feeding and gas exchange. In terrestrial vertebrates, they don't persist as slits but instead develop into other structures like Eustachian tubes and tonsils.
Dorsal hollow nerve cord: Formed from a plate of ectoderm that rolls into a tube, located dorsal to (above) the notochord. The anterior end develops into the brain, and the posterior portion becomes the spinal cord in vertebrates. This is different from invertebrates like arthropods, which have a ventral solid nerve cord.
Post-anal tail: A muscular extension of the body posterior to the anus, used for locomotion in aquatic species. Humans have this during embryonic development (it regresses into the coccyx/tailbone).
Chordates also have a segmented muscular system with blocks of muscle called myomeres arranged along the notochord, enabling the side-to-side swimming motion you see in fish and larval amphibians.
Embryonic Development in Deuterostomes
During gastrulation, three primary germ layers form, each giving rise to specific tissue types:
- Endoderm (innermost): lining of the digestive tract and associated organs (liver, pancreas, lungs)
- Mesoderm (middle): muscles, skeleton, circulatory system, and kidneys
- Ectoderm (outermost): nervous system and outer skin (epidermis)
Early deuterostome embryos display bilateral symmetry. Most chordates maintain this bilateral symmetry into adulthood, while echinoderms shift to pentaradial symmetry after metamorphosis. This contrast makes echinoderms a useful reminder that adult body plan and embryonic development can tell very different evolutionary stories.