Phylum Ctenophora PDF

# Chapter 10: Phylum Ctenophora The phylum Ctenopora includes a small group of free-swimming or planktonic marine animals with transparent, delicate, gelatinous bodies. They generally resemble the coelenterates and are often grouped with them in some classifications. They are even more nearly trans...

# Chapter 10: Phylum Ctenophora The phylum Ctenopora includes a small group of free-swimming or planktonic marine animals with transparent, delicate, gelatinous bodies. They generally resemble the coelenterates and are often grouped with them in some classifications. They are even more nearly transparent than the coelenterate jelly-fish and often termed as the comb-jellies or sea-walnuts. They have been used in various experimental work, but they have no direct economic value. ## History The ctenophores do not have as long a recorded history as the coelenterates. Probably the common forms were known to ancient people. But the first definite description of ctenophores was given by a ships's doctor, Martens (1671). Linnaeus and Cuvier placed some ctenophores under Zoophyta, class Acalephae. Eschscholtz (1829-1833) arranged the pelagic coelenterates under three orders-Ctenophorae, Discophorae and Siphonophorae, thus, recognizing the ctenophores as a distinct group for the first time. In 1847, Leuckart separated from echinoderms the phylum Coelenterata which also included sponges and ctenophores. It was Hatschek (1839) who put all the ctenophores under a distinct phylum. ## Habits and Habitat Ctenophores are very common marine animals often occurring in enormous schools. They are widely distributed, being especially abundant in the warmer seas, though some occur in temperate or arctic regions and a number of species are cosmopolitan. They are of planktonic habit, floating in the surface waters, mostly near shores, but a few live up to a depth of even 3,000 meters leading a benthonic or creeping existence. They rest vertically in the water and are propelled but feebly by the rhythmic wave-like movements of the paddle plates. Being feeble swimmers, they are carried about by currents and tides, often accumulating in vast numbers. Ctenophores are noted for their delicacy and beauty. When seen in sunlight their vibrating comb-plates, refracting the light, show a rapid play of changing colours, giving the effect of successive series of rainbow colours. Many are highly phosphorescent in the dark. Production of light occurs in the walls of the meridional canals, but externally, the comb-plates appear to emanate light. Ctenophores are voracious carnivores feeding on plankton. They feed on small marine animals including the eggs and larvae of molluscs, crustaceans and fish. As the animal swims, the tentacles, trailing behind the body, capture the food that comes in contact with them. ## External Features The general plan of body resembles that of a coelenterate medusa. ### Shape A typical ctenophore is somewhat spherical, pear-shaped or cylindrical in shape. They are commonly termed gooseberries or sea walnuts because of their shape. The outer surface is without hard skeletal parts. The spherical body can be divided into two hemispheres. The mouth lies at one end (oral) of the body and a sense organ at the opposite end or pole (aboral) of the body. ### Symmetry A ctenophore is said to possess biradial symmetry. The parts of the body are radially disposed but lie half on one side and half on the other side of a median longitudinal plane or oral-aboral axis. Thus, there are two planes-saggittal and transverse-along which the body can be divided into two exactly similar halves. Thus, structures are tetramerously arranged in a radial fashion around the oral-aboral axis. ### Size Ctenophores are of moderate size, ranging from a few millimeters to 20 cm. but a few species of _Cestum_ reach up to 1 meter or more in length. ### Color Ctenophores are usually transparent, but various structures such as the tentacles and the comb-rows are tinged with white, orange or purple. ### Comb-plates A ctenophore swims about in water with the help of 8 equally spaced rows of paddle plates, arranged on the sides of the body like meridians from pole to pole on a globe, thus further dividing the body into 8 equal sections. Each paddle plate is a slight ridge composed of a transverse band of long fused cilia. When viewed from side, the paddle plates appear like the teeth of a comb, so that they are termed the comb-plates or ctenes, and the ctenophores commonly named the comb-jellies. The rows of ciliated bands form roughened ridges, similar to those on a walnut, hence the other common name sea-walnuts for the ctenophores. The comb rows may be variously tinged with white, orange or purple. ### Tentacles Nearer the aboral end, on opposite sides of the body are two long, solid and retractile tentacles, each projecting from a deep blind pouch or tentacle sheath into which it can be wholly or partially withdrawn. Each tentacle bears a row of short branches or pinnae in a single row. The tentacles are highly contractile, each with a mesenchymal core covered by assp cells or colloblasts. ### Nematocysts Nematocysts are absent; instead, the tentacles are armed with peculiar adhesive cells called lasso cells or colloblasts, which are quite different from the nematocysts in structure and function. A colloblast is a modified epidermal cell. It has a hemispherical head, situated on the surface and connected to the central mesenchymal core by a stiff, non-elastic, straight filament, which originally was the nucleus of the cell. An elastic, spring like spiral filament is coiled around the straight filament. The surface of the head is covered with secretory granules, which produce an adhesive material to entangle and kill small organisms which are then conveyed to the mouth. The colloblasts cannot sting as they lack the mechanism for penetration. They are absent in at least one ctenophore, _Euchlora rubra_, which however, possesses nematocysts, thus reflecting a common origin of Ctenophora with Coelenterata in the distant past. ### Sensory organ At the aboral end of the body is a deep pit or concavity lined by tall, ciliated, sensory epithelial cells. It is the statocyst which is an organ of equilibrium and forms an outstanding peculiarity of the ctenophores. Its centre is occupied by a small rounded mass of calcareous particles, the statolith, which is supported by four tufts of long fused cilia, called balancers. The statocyst is covered by a roof, like a bell dome, formed of fused cilia. Eight ciliated furrows lead out from the sense organ to the eight rows of comb-plates. Two furrows extend from each balancer to the two comb rows of that quadrant of the animal. Two ciliated tracts or polar fields extend outwards, in the sagittal plane, from two opposite sides of the statocysts. A strong sphincter, surrounding the sense organ, serves to close off the statocyst. The sensory organ serves as an organ of equilibrium. If the body is inclined, the statolith presses more heavily the ciliary tuft of the inclined side and the stimulus is carried by the ciliated furrow to the comb plates which beat faster, thus righting the animal. If a ciliary furrow is severed, the beating of the corresponding comb rows becomes independent and no longer coordinated with that of the other rows. Undigested food passes out through either the mouth or the anal pores. ## Internal Anatomy ### Histology The body is triploblastic. The outer epidermis is syncytial in some forms but usually made of cuboid or columnar cells. It often contains sensory, glandular, mucous and pigment cells, but nematocysts are absent except in a single species (_Euchlora rubra_).