Chordates belong to animal phylum Chordata and comprise vertebrates, together with many closely related invertebrates, urochordates and cephalochordates. Until recently, the invertebrate group, Hemichordata was put under phylum Chordata but is now considered as the separate phylum. Three chordate subphyla that is Urochordata, Cephalochordata and Vertebrata are so grouped on account of containing certain main characteristics i.e.: a) notochord (dorsal fairly rigid rod of vacuolated cells) or vertebral column b) hollow dorsal nerve cord or spinal cord c) pharyngeal slits d) post-anal tail.
Characteristics of Chordates:
General Classification of Chordates:
Phylum Chordata made of four subphyla and every subphylum is composed of classes. Every class in turn is composed of orders. Classify protochordates to level of class only and attempt to classify the vertebrates a step further down to the level of order.
• Subphylum Cephalochordata (head chordates)
The hemichordates are considered as half chordates as their chordate characteristics are incomplete or not well-developed, and they don't have post-anal tail. Urochordates are noted for the tail (uro-) while cephalochordates illustrate some degree of head (cephalo-) formation. In vertebrates, notochord has been replaced by the true backbone/vertebral column, composed of either cartilage or bone. Examine the different subgroups in each subphylum.
Subphylum Hemichordata (half chordates)
Class Enteropneusta (acorn worms)
Class Planctosphaeroidea (extinct)
Class Pterobranchia (Cephalodiscus)
Subphylum Urochordata (tail chordates)
Class Ascidiacea (sea squirts) Class Thaliacea (salps)
Class Larvacea (Appendicularia)
Subphylum Cephalochordata (tail chordate)
Class Leptocardii (Leptocardia
Three invertebrate subphyla (Hemichordata, Urochordata, Cephalochordata) given are jointly known as protochordates. They indicate primitive form of chordates.
Other classification of the subphylum Vertebrata and the subgroups under it is also given.
The subphylum Hemichordata has the following characteristics:
The hemichordates are distinguished by the tripartite (three parts) division of body. At anterior end of body is the proboscis (pre-oral lobe); behind this are collar, and finally a trunk. Hemichordates share some of typical chordate characteristics. They contain bronchial openings, or gill slits, which open in pharynx; there is rudimentary structure in collar region known as stomochord, or diverticulum (blind sac) which is composed of cells which resemble those found in notochord but which is not true notochord. There is dorsal nerve cord, additionally to smaller ventral nerve cord.
Characteristics of Class Enteropneusta (Acorn worms):
Enteropneusts included the majority of hemichordates with more than 70 species. They are typical acorn worms and extremely well fit explanation of hemichordates above. They contain the following features:
Enteropneusta (acorn worms) are 2 to 2.5m long; marine in shallow waters, lonely, dwell in mud or vegetation; filter-feeders. They contain well-developed gill slits, and stomochord. They also contain a dorsal strand of nerve cells, thought to be precursor to dorsal hollow nerve cord. Enteropneustes are slow burrowers.
Characteristics of the Class Pterobranchia:
The class is characterized by the given characteristics:
In Pterobranchia (pterobranchs) there is no trace of the dorsal nerve cord or notochord; have only one pair of gill slits in species of the genus Cephalodiscus. Pterobranchs are the doubtful group of animals, which unlike acorn worms; create colonies in which individuals are interrelated by stems, or stolons. Proboscis is not lengthened, as in acorn worms, but shield- shaped. Collar bears a pair of branched tentacles which collect small food particles from water.
Features of Subphylum Urochordata (Tunicata):
Urochordates are medium sized group of marine animals usually referred to as Sea squirts, Tunicates, Salps or Larvaceans. They are all filter feeders using the essentially similar mechanism of pumping water by the perforated pharynx that gathers small particles in layer of mucus. All urochordates contain external covering or 'house' known as tunic that is composed of secreted proteins and polysaccharide much like cellulose. Subphylum is divided in 3 classes: Ascidiacea, Thaliacea and Larvacea.
Characteristics of the Class Ascidiacea:
This class is signified by Sea squirts and they make up bulk of species found within phylum Urochordata. The features of Class Ascidiacea are given below:
Tunicate larvae don't feed and are fundamentally a dispersal form. They soon find the appropriate spot on sea floor and settle in head down, tail up position. They join themselves to sea floor (substrate) through special adhesive glands in front of head and then suffer amazing metamorphosis in which post-anal tail and notochord are lost. Tunicates feed by drawing water in by inhalant siphon. Water passes by pharynx where small particles are trapped before water leaves body by exhalent siphon. Mucous is secreted by special cells and is moved across surface of pharynx by beating of several small cilia, finally it is passed in digestive tract where both particles and it caught up in it are digested.
Features of Class Thaliacea (salps):
Characteristics of the class Thaliacea are given below:
The class has approx 70 species. Between two siphons, water passes by several pore or slats of enlarged pharynx that occupies most of body cavity. Tiny particles of plankton are gathered on film of mucus that constantly passes across pharynx. Mucus is secreted by special cells and is kept on moving through beating of many small cilia until it is swept in digestive tract.
Characteristics of Class Larvacea (Apendicularia):
The Larvaceans, at times known as apendicularians, are small animals and are fairly different in form to rest of Urochordata.
Their characteristics are given below:
3) Cephalochordata Subphylum Cephalochordata:
This class comprises several species of lancelets, or amphioxi, tiny, fishlike, filter-feeding animals discovered in shallow water. The lancelet contains the long body, pointed at both ends, with large notochord which extends almost from tip to tip and is there throughout life. It has closed blood circulatory system.
Class: Leptocardii/Leptocardia (small heart):
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