Subphylum Cephalochordata
Subphylum
Cephalochordata
Cephalochordates are the marine lancelets: slender, laterally compressed, translucent animals about 5 to 7 cm in length (Figure 25-10) that inhabit the sandy bottoms of coastal waters around the world. Lancelets originally bore the generic name Amphioxus (Gr. amphi, both ends, + oxys, sharp), later surrendered by priority to Branchiostoma (Gr. branchia, gills, + stoma, mouth). Amphioxus is still used, however, as a convenient common name for all of the approximately 25 species in this diminutive subphylum. Four species of amphioxus are found in North American coastal waters.
Amphioxus is especially interesting because it has the four distinctive characteristics of chordates in simple form. Water enters the mouth, driven by cilia in the buccal cavity, then passes through numerous pharyngeal slits where food is trapped in mucus, which is then moved by cilia into the intestine. Here the smallest food particles are separated from the mucus and passed into the hepatic cecum (liver diverticulum) where they are phagocytized and digested intracellularly. As in tunicates, the filtered water passes first into an atrium, then leaves the body by an atriopore (equivalent to the excurrent siphon of tunicates).
The closed circulatory system is complex for so simple a chordate. The flow pattern is remarkably similar to that of primitive fishes, although there is no heart. Blood is pumped forward in the ventral aorta by peristaltic-like contractions of the vessel wall, then passes upward through branchial arteries (aortic arches) in the pharyngeal bars to paired dorsal aortas which join to become a single dorsal aorta. From here the blood is distributed to the body tissues by microcirculation and then is collected in veins, which return it to the ventral aorta. Lacking both erythrocytes and hemoglobin, their blood is thought to transport nutrients but play little role in gas exchange.
The nervous system is centered around a hollow nerve cord lying above the notochord. Pairs of spinal nerve roots emerge at each trunk myomeric (muscle) segment. Sense organs are simple, unpaired bipolar receptors located in various parts of the body. The “brain” is a simple vesicle at the anterior end of the nerve cord.
Sexes are separate. The sex cells are set free in the atrial cavity, then pass out the atriopore to the outside where fertilization occurs. Cleavage is total (holoblastic) and a gastrula is formed by invagination. The larvae hatch soon after deposition and gradually assume the shape of adults.
No other chordate shows the basic diagnostic chordate characteristics as clearly as amphioxus. In addition to the four chordate anatomical hallmarks, amphioxus possesses several structural features that suggest the vertebrate plan. Among these are a hepatic cecum, a diverticulum that resembles the vertebrate pancreas in secreting digestive enzymes, segmented trunk musculature, and the basic circulatory plan of more advanced chordates. As discussed later, many zoologists consider amphioxus a living descendant of an ancestor that gave rise to both the cephalochordates and the vertebrates. Therefore cephalochordates are, in cladistic terms, the sister group of the vertebrates (Fig-ure 25-3).
Cephalochordates are the marine lancelets: slender, laterally compressed, translucent animals about 5 to 7 cm in length (Figure 25-10) that inhabit the sandy bottoms of coastal waters around the world. Lancelets originally bore the generic name Amphioxus (Gr. amphi, both ends, + oxys, sharp), later surrendered by priority to Branchiostoma (Gr. branchia, gills, + stoma, mouth). Amphioxus is still used, however, as a convenient common name for all of the approximately 25 species in this diminutive subphylum. Four species of amphioxus are found in North American coastal waters.
Amphioxus is especially interesting because it has the four distinctive characteristics of chordates in simple form. Water enters the mouth, driven by cilia in the buccal cavity, then passes through numerous pharyngeal slits where food is trapped in mucus, which is then moved by cilia into the intestine. Here the smallest food particles are separated from the mucus and passed into the hepatic cecum (liver diverticulum) where they are phagocytized and digested intracellularly. As in tunicates, the filtered water passes first into an atrium, then leaves the body by an atriopore (equivalent to the excurrent siphon of tunicates).
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| Figure 25-10 Amphioxus. This interesting bottom-dwelling cephalochordate illustrates the four distinctive chordate characteristics (notochord, dorsal nerve cord, pharyngeal slits, and postanal tail). The vertebrate ancestor is thought to have had a similar body plan. A, Internal structure. B, Living amphioxus in typical position for filter feeding. Note the oral hood with tentacles surrounding the mouth. |
The closed circulatory system is complex for so simple a chordate. The flow pattern is remarkably similar to that of primitive fishes, although there is no heart. Blood is pumped forward in the ventral aorta by peristaltic-like contractions of the vessel wall, then passes upward through branchial arteries (aortic arches) in the pharyngeal bars to paired dorsal aortas which join to become a single dorsal aorta. From here the blood is distributed to the body tissues by microcirculation and then is collected in veins, which return it to the ventral aorta. Lacking both erythrocytes and hemoglobin, their blood is thought to transport nutrients but play little role in gas exchange.
The nervous system is centered around a hollow nerve cord lying above the notochord. Pairs of spinal nerve roots emerge at each trunk myomeric (muscle) segment. Sense organs are simple, unpaired bipolar receptors located in various parts of the body. The “brain” is a simple vesicle at the anterior end of the nerve cord.
Sexes are separate. The sex cells are set free in the atrial cavity, then pass out the atriopore to the outside where fertilization occurs. Cleavage is total (holoblastic) and a gastrula is formed by invagination. The larvae hatch soon after deposition and gradually assume the shape of adults.
No other chordate shows the basic diagnostic chordate characteristics as clearly as amphioxus. In addition to the four chordate anatomical hallmarks, amphioxus possesses several structural features that suggest the vertebrate plan. Among these are a hepatic cecum, a diverticulum that resembles the vertebrate pancreas in secreting digestive enzymes, segmented trunk musculature, and the basic circulatory plan of more advanced chordates. As discussed later, many zoologists consider amphioxus a living descendant of an ancestor that gave rise to both the cephalochordates and the vertebrates. Therefore cephalochordates are, in cladistic terms, the sister group of the vertebrates (Fig-ure 25-3).
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