9.4 (a) Phylum Hemichordata (affinities)
Hemichordates closely resemble both echinoderms and chordates because only these three groups are deuterostomes. So far only 70 species of them are recorded.
Hemichordates were earlier placed in chordates as a group, but now they are placed in a separate phylum. Although these animals have some chordate-like characteristics such as deuterostome, pharyngeal gill slits, and a dorsal nerve cord (sometimes may be hollow). However, lacks a complete chordate-like notochord, The Blood vascular system is non-c
chordate i.e., the dorsal heart and, the epidermal nervous system like non-chordate characters. Thus given an independent phylum of its own named hemichordate, and placed at the top of invertebrate phyla
9.4 (b) General characteristics of Hemichordates (Tongue worms)
- Their body is soft and unsegmented and mostly worm-like in shape.
- The body can be distinguished into proboscis, collar, and trunk.
- The epidermis contains mucus-secreting cells.
- Bilaterally symmetrical and triploblastic. Coelom consists of three portions. Gills are their respiratory organs. Blood is colorless and without cells.
- A dorsal heart has an invertebrate-like blood vessel.
- Well-developed excretory system (glomerulus is present).
- Epidermal nervous system.
- These are either unisexual or bisexual.
- Fertilization is an external, indirect development. things.
- Examples: Balanogving.
Habits:
Marine, either solitary or colonial, free-living or fixed.
Phylum Chordata:
The most successful, well-known, and widely distributed animals are chordates. Chordates exhibit great diversity of form, habitat, and habit. This phylum is divided mainly into two groups, invertebrate chordates (acrania)
9.4.1 Fundamental or Basic Characteristics of Chordates
All chordates possess three basic or fundamental characteristics, during some stages or in their whole life that is the notochord, dorsal hollow nervous system, gill slits, and as a fourth character, and sometimes tail is also present.
Notochord is an unjointed solid skeleton, placed above the alimentary canal and below the dorsal body wall, and the central nervous system appears in the embryo of all chordates. In invertebrate chordates, it is present as such throughout life but in the vertebrates replaced into the vertebral column, which is segmented. Serves as an axial endoskeleton and gives support to the body.
Dorsal hollow nervous system:
The central nervous system in chordates is dorsally placed, located above the notochord. It is a hollow and fluid-filled and nonganglionated nerve cord. Pharyngeal Gill slits and gill pouches are paired sets of openings in the pharyngeal region, in aquatic chordates it persists and functions as a respiratory organ but in terrestrial vertebrates it is replaced by Eustachian or auditory tube, Parathyroid, tonsils, and thymus.
9.4.2 Invertebrate Chordates (Acrania)
The term Acrania means "without skull" This division includes animals with the following characteristics: These are without brain and cranium (brain box). Jaws, RBC, and paired appendages are absent. Notochord never changed into the vertebral column. This group consists of two subphyla.
Subphylum Urochordata:
In this subphylum, the notochord is present only in the tail region of larva while disappearing in adults. The nerve cord also disappeared in adults, only dorsal ganglion is present. The larvae are free swimming while adults are nonmotile. The body is covered with a tunic (test). therefore also called tunicata e.g., ascidia, herdmania, molgula.
Subphylum Cephalochordata (Cephalo; head):
Notochord runs mid-dorsally throughout the body. Notochord and nerve cord persist throughout life. Example Amphioxus (Branchiostoma).
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