Tunicate

With some general characteristics covered, we are now beginning our journey through phylum Chordata, which will take us through an incredible variety of organisms.

To start things off, within the phylum Chordata, there are three extant subphyla, Cephalochordata, which contains the lancelets, Tunicata, which contains the tunicates, and Vertebrata, which contains the vertebrates.

Remember that all members of phylum Chordata share the five chordate characteristics we mentioned at least some point in their lives, those being the notochord, dorsal tubular nerve cord, pharyngeal pouches or gill slits, endostyle, and postanal tail.

So let's begin our exploration of the chordates with subphylum Cephalochordata, the sister clade to the olfactories.

They were traditionally classified as the sister group to the vertebrates due to similar morphological features, but extensive molecular phylogenetic analyses have revealed that the cephalochordates are actually the most basal of the chordates, and the tunicates are the sister group to the vertebrates, meaning the tunicates have secondarily lost some chordate characteristics.

And that's a bit of information on the cephalochordates.

In the next tutorial, we will examine their sister group, the olfactories, and the split between clades Tunicata and Vertebrata.

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The body of a tunicate is enclosed in a thick, rubbery outer covering called a tunic, which gives them their common name.

The tunic is composed of a carbohydrate called cellulose, which provides support and protection for the animal.

Tunicates can range in size from just a few millimeters to more than a meter in length, depending on the species.

They are filter feeders that primarily feed on plankton and other small particles in the water.

Tunicates have an incurrent siphon through which they filter food and an excurrent siphon through which they expel waste.

Their digestive system begins when water that enters through the incurrent siphon passes through the cilia-lined pharynx and pharyngeal basket, which contains the pharyngeal slits and endostyle that trap food and transport it to the stomach.

Their circulatory system consists of a ventral heart and two large vessels that branch into the pharyngeal basket that also functions in gas exchange as well as digestion.

Their nervous system is comprised of a simple nerve ganglion and nerve groups, and most sensory organs are absent in adults.

Brachiopoda, provided with two ciliated arms which they have in place of them.

Tunicata, unprovided with arms and special organs of motion.

Cuvier and many other zoologists arrange the seropoda also in the branch of mollusca, but it is now ascertained that these animals belong to the branch of articulata, the third great division of the animal kingdom.

End of Lesson 1 Recording by Lauren Huff Lesson 2 of Elements of Conchology This is a LibriVox recording.

Class of Mollusca Brachypoda.

General characters, Lingulo, Terebrachula, Orbicula.

Class of Mollusca Tunicata or Acephala Nuda.

Bifora, Ascidia, Botryllus, Pyrosoma.