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Kingdom Animalia
Phylum Chordata
Phylum Chordata
(Canadian Campbell 2nd ed. Concept 34.1)
Although we are primarily interested in the vertebrates in this part of the course, we must recognize that there are various animal types lacking a backbone but clearly allied to the vertebrates.
The lower chordates lack a backbone and many other advanced structures possessed by their vertebrate relatives. They do, however, exhibit three basic features characteristic of the vertebrates:
The phylum Chordata is usually subdivided into the three subphyla which may be listed as follows in a roughly ascending order of complexity:
This group of organisms is commonly referred to as the sea squirts, and shows the basic characteristics (gill pouches (slits), notochord, dorsal nerve cord) of the Chordates. These are rather common small, sessile marine organisms. As an adult they appear as an almost formless lump attached to a rock or other underwater object and covered with a leathery looking "tunic". The only structural features seen externally are an opening at the top, into which water passes, and another lateral opening through which the water current flows outward. The animal shows no external resemblances to the vertebrates, and internally much of the structure is equally unfamiliar. There is no notochord, nor is there a nerve cord. Instead, there is a simple nerve ganglion with a few nerves splaying out from it. Much of the interior of the animal is occupied by a barrel-shaped structure which serves as the food-gathering device. The barrel is an exaggerated set of internal gills. We have in the pharyngeal gill apparatus, a high degree of development of one of the primary characters to be sought in a relative of the vertebrates. For other characters we must look at the larval stage of some tunicates. The larval form resembles an amphibian tadpole. In the tail there is a well-developed notochord and a typical dorsal hollow nerve cord. Anteriorly in the larva there is a rudimentary brain and sense organs. Once the larva attaches and becomes a sedentary adult, the notochord and larval nervous system disappears.
Subphylum
Cephalochordata
(Canadian Campbell 2nd ed. Page 761, Fig 34.4)
In this subphylum the similarity of Amphioxus to the vertebrates is most obvious. Amphioxus is a small, translucent animal, fishlike in appearance and proportions, found in shallow marine waters in various regions of the world. As its shape suggests, it can swim readily, but spends most of its time with its body buried in the sands, with only its anterior end projecting. The main skeletal structure is a highly developed notochord, which persists throughout life and (in contrast to the vertebrate situation) extends to the tip of the "nose", a feature to which the group owes its name. The notochord prevents telescoping of the body during vigorous swimming and serves as a convenient central "peg" on which to hang body organs. There is a hollow dorsal nerve cord, but no true brain, and a digestive tract consisting of a straight tube. The pharynx is highly specialized for food collecting. However, while the gill slits, by their major development, emphasize the animal's relationships to the vertebrates, they differ from the vertebrate gill apparatus both in the purpose for which they are utilized and their mode of operation. It appears that most of the respiration of Amphioxus is done through its thin skin and as we have discussed in other lower chordates, the gills are feeding devices. Further, water currents through the gills in vertebrates are effected by muscular pumping, in Amphioxus ciliated action alone is responsible.
Examine the Amphioxus: Examine the Whole mounts of Amphioxus and note the general body form; in particular note the gill slits.
Examine the cross section of Amphioxus through the pharyngeal region and note: gill slits, notochord and dorsal hollow nerve cord.
