Invertebrate Chordates © Lisa Michalek

Animal Development As an embryo develops, it goes through a gastrula stage. A gastrula has an opening to the outside called the blastopore. In acoelomate animals, the mouth develops from or near the blastopore. This pattern of development also occurs in some coelomate animals, such as annelids, mollusks, and arthropods. Animals with mouths that develop from or near the blastopore are called protostomes.

Animal Development Some animals follow a different pattern of development. In phylums Echinodermata and Chordata, the anus (not the mouth) develops from or near the blastopore. Animals with this pattern of development are called deuterostomes.

Protostome vs. Deuterostome Development

Animal Development The first deuterostomes were marine echinoderms that evolved more than 650 million years ago. They were also the first animals to develop an endoskeleton. Today, most people are familiar with echinoderms known as “starfish,” which are not really fish and are more properly called sea stars. In addition to the sea stars, many other animals commonly seen along the sea shore (sea urchins, sand dollars, and sea cucumbers) are echinoderms. All are marine, and all are radially symmetrical as adults.

Animal Development Chordates, as well as a few other small phyla, are also deuterostomes. Like the echinoderms, chordates have an internal skeleton. This developmental similarity unites these seemingly dissimilar animal phyla It also leads scientists to believe that chordates and echinoderms derived from a common ancestor. The identity of the ancestral deuterostome is not known. The fossil record indicates that echinoderms, such as the sea lily were abundant in the ancient seas.

Invertebrate Chordates The second major group of deuterostomes are the chordates. Chordates have a very different kind of endoskeleton from that of echinoderms. The chordate endoskeleton is completely internal.

The Chordate Skeleton During the development of the chordate embryo, a stiff rod called the notochord develops along the back of the embryo. Using muscles attached to this rod, early chordates could swing their backs from side to side, enabling them to swim through the water. The development of an internal skeleton was an important step that led to the evolution of vertebrates. The endoskeleton, which muscles attach to, made it possible for animals to grow large and to move quickly.

Other Chordate Characteristics Chordates also share three other characteristics. They have a single, hollow, dorsal nerve cord with nerves attached to it that travel to different parts of the body.

Other Chordate Characteristics Chordates also have a series of pharyngeal pouches. Pharyngeal pouches of aquatic chordates develop in the wall of the pharynx and develop into the gill structures later in the animals’ development. In terrestrial chordates, the pharyngeal pouches develop into different structures, such as the parathyroid gland and the inner ear.

Other Chordate Characteristics Another chordate characteristic is a postanal tail, which is a tail that extends beyond the anus. All chordates have all four of these characteristics at some time in their life, even if it is only briefly as embryos.

Invertebrate Chordates Phylum Chordata is divided into three subphyla. The majority of chordate species belong to subphylum Vertebrata. Two other subphyla, Urochordata (the tunicates) and Cephalochordata (the lancelets), contains a small number of species. Because members of these two subphyla are chordates that do not have backbones, they are called invertebrate chordates.

Tunicates Only the free-swimming tunicate larvae have a nerve cord, notochord, and postanal tail. These features are lost during the larvae’s transformation into adulthood. However, adult tunicates retain their pharyngeal slits. Most adult tunicates are sessile, filter- feeding marine animals.

Tunicates A tough sac, called a tunic, develops around the adult’s body and gives tunicates their name. Cilia beating within the tunicate cause water to enter the incurrent siphon. The water circulates through the tunicate’s body, passes through the pharyngeal slits, and leaves the body through the excurrent siphon.

Tunicates As water passes through the slits in the pharynx, food is filtered from it and passed into the stomach. Undigested food passes to the anus, which empties into the excurrent siphon. All tunicates are hermaphrodites, and some are also able to reproduce asexually by budding. While some tunicates are solitary, budding can result in colonies of identical tunicates.

Lancelets Lancelets receive their name from their bladelike shape. Although lancelets may resemble fish, they are not fish. Lancelet fossils have been found in rocks over 550 million years old. They are much older than any fish species.

Lancelets Lancelets are scaleless chordates only a few centimeters long. The lancelets’ V-shaped bundles of muscles are arranged in a series of repeating segments. Lancelets are found worldwide in shallow ocean water. They spend most of their time with their mouths protruding from mud or sand.

Lancelets The beating cilia that line the front end of their digestive tract draws water through the mouth and pharynx and out the pharyngeal slits. Lancelets feed on microscopic protists that they filter out of the water. Unlike tunicates, the sexes are separate in lancelets.