Platyhelminthes: India
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Platyhelminthes
Introduction
The Platyhelminthes or flat worms constitute one of the major phyla of Animal Kingdom. These animals exhibit a great diversity of form and diverse bionomic relations.
They are included in a single phylum due to certain common features, viz. bilateral symmetry, dorsoventral flattening of body, ciliated epiderims in free living forms and at some stage in parasitic species, absence of coelome or body cavity, an incomplete digestive system, excretory system formed of protonephridia or flame-cells. One or more of these characters may be vestigeal or lacking in parasitic forms. They also have certain common negative features such as absence of metamerism, skeletal, circulatory and respiratory systems. Hyman (1951) defined them as "A coelom ate Bilatoria with a definite anus". Fonnerly they were divided into three classes viz. Turbellaria, Trematoda and Cestoda. Some authors include Mesozoa and Nemertea also under this phylum, while majority consider them separate phyla. In recent times class Tre~atoda has been divided into two separate classes viz. Monogenea and Trematoda.
Most of the members 9f the phylum are parasitic and this phenomenon has greatly influenced the character of the group~ Within the phylum there are groups illustrating gradual transition from a free-living mode of life to extreme dependence. The organs useful for parasitic mode of life have been proliferated while those structures which are essential for free living existence have been reduced or discarded. Thus we find absence of locomotary and sensory organs. The absorption of nutrients by the body surface has resulted in progressive regression of digestive system, so much so that in cestodes it is entirely absent.
The metabolic rate is probably not affected and efficient excretory system has developed for discharging nitrogenous waste. Their location inside the host body with limited supply of free oxygen led them to adapt to araerobic respiratfon. Nonnally the parasite remains within and perishes with the host. For perpetuation of the species the progeny must escape from the first host and find a new host. The larval stages are minute with little capacity to take food and limited locomotive capacity. To cope up with this hazards of parasitism these groups have developed enormous reproductive capacity and interpolated alternative hosts.
The phylum Platyhelminthes comprises four classes: Turbcllaria, Monogenea, Trematoda, and Cestoda.
Turbellaria
Turbellarians are generally small, free living carnivorous worms. Mostly they are less than 5 mm in length but the larger forms also occur in the orders Tricladia and Polycladida; some of the terrestrial planarians attain a length upto 50 ems. They are oval to elongate, flattened ventrally and somewhat convex dorsally. The body may be tuberculate or papillate; members of the Temnocephalida have anterior tentacles and a ventral muscular adhesive organ. The smaller species are white or translucent and present shades of gray or brown depending on the ingested food. The larger species are often brilliantly coloured due to presence of pigments in or under the epidermis, and have stripes, bars, or blotched patterns in grecn, red, yellow, orange and black colours. The anterior cnd bears the sensory organ, contains the principal ganglionic mass or brain of nervous system and precedes in locomotion.
The body is covered by a cellular or syncytscal epidermis beset with cilia or minute rhabdoral spiCUles. They arc primarily hermaphroditic; with often complex reproductive system. Thc fertilization is internal and life cycle simple. The cggs usually contain large amount of food material in the form of vistellinc cells, but the Acocla and Polycladida lack vitelline glands and in some of the polyclads a free swirnlning larva is produced. Many of the Turbellaria are either commensals or parasites : the rhabdocoels in echinoderms and molluscs; the allocoels in Crustaceans; the triclads in crustaceans, echinoderms, molluscs, chelicerate arthropods and selachian fishes.
They are said to be the precursors of different classes of the phylum Platyhelminthes. The classification of the group is based on the details of digestive or reproductive systems by different workers. The most accepted classification is based on the fonn of intestine and comprises the following five orders:
Acoela Minute,. marine forms, mouth ventral, no distinct intestine, gonad in parenchyma, without reproductive or excretory ducts~ at times coloured green or brown due to symbiotic algae.
Rhabdocoela Small, marine, freshwater or terrestrial forms, free living, commensal or parasitic worms, pharynx simple, intestine saccate, gonads compact, testes few, reproductive ducts and simple protonephridial excretory system.
Allococla Usually larger than rhabdcoels, mostly marine, a few freshwater fonns, pharynx of variable type, intestine lobate, testes numerous, prostonephridial system complex.
Tricladida Large, elongate marine fresh. Water or terrestrial worms. Pharynx plicate, intestine triclad, Gonopore single. Excretory system complicated with numerous nephridiopores.
Polycladida Large, broad, typically marine forms of shrinking shape and colouration. Pharyrx plicate, intestine much branched, gonads numerous, vitellaria absent; gonadapore cOlnmon or separate for male and female ducts.
Historical Resume
The foundation for a sound knowledgc of Indian Turbellarians was laid in the rust quarter of the 20th century by workcrs such as White-house, Mcxiner. Muth. Kaburaki, DUlta and others. Unfortunately the trend has not been maintained in recent years. Except for a few occasional papers the group appears to have received vcry lilllc attention.
Mexiner and Muth (1912) reported on a collection of aquatic turbellarians, made in Tibet by Capt. F. H. Stewart, during the year 1907. They dealt with the description of certain species of the families Catcnulidae, Dalydcllidae, Typhloplanidae under the Rhabdocoela, and Sorocelis under the Tricladida
Whitehouse (1913) described two new species of planarians, viz. Planaria aborensis and P. kempi from the Abor Hills in the North-east India. Again in 1914, he described eight new species of land turbcllarians of the genera Bipalium. Placocephalus. Pelmatoplana and Cotyloplana. Further, Whitehouse (1918) gave a detailed systematic account of Indian land turbellarians in the possession of Indian Museum.
The collections were earlier made from several parts of former Travancore and Cochin states; Nilgiri Hills; Coimbatorc in south India; Kumaon Hills in the Western Himalayas; Kurseong, Cherrapunji (Meghalaya), in thc Eastern Himalayas; and some from Sri Lanka. In this interesting paper he dealt with 11 species of Bipalium of which six were described as, new to science. Further, new species of COlyloplana from Nilgiris and four new species of Pelmatoplana from Kumaon, Kurseong, Pallipola (Sri Lanka) and Madathoray (fonner Travancore state) were described at length.
Based on a very small collection of freshwater and land planarians from Andaman Islands, Kabursaki (1925) dealt with new species, viz. Planaria andamanensis and Bipalium vinosum. Besides, Kabusami (1918, 1920) also contributed to our knowledge on the freshwater triclad fauna of adjacent countries such as Inle Lake in Burma and Siam in Thailand. Arora (1944) recorded freshwater turbcllaria Croln Kashmir and Kapadia (1947) the occurrence of Bipalium sp. in Junagadh (Kathiawar, Gujrat State). Ramakrishna (1953) reported for the first lime occurr~nce of land
Platyhelminthes
Planarian, Dolichoplanafeildeni from India. Saxena (1957) gave an account of a new species of Bipaluim viz. B. keshavi, from Nepal. Chauhan and Ramakrishna (1958) dealt with description of anew turbellarian Bipalium roonwali. Kawakatsu (1969) described Dugesia indica from Jabalpur. Ramakrishna Rao (1987) studied the systematic and the biochemical contents of polyclad worms from Waltair coast
Very little attention seems to have been paid to other aspects of study, viz., anatomy, bionomics, physiology etc. Dutta (1925) dealt with the anatomy of a rhabdocoelid turbellaria, Mesostoma gangelica and he (1926) described the structure and bionomics of a freshwater turbellaria belonging to Acoela. Ghoshal (1988) gave an account of regeneration in a freshwater turbellarian. Much remains to be done in this group and there is a need to develop expertise in the country.
Estimation of Taxa
C. B. Srivastava, Zoological Survey of India, 234/4, AJ.C. Bose Raod, Calcutta -700 020.
Elsewhere
G. Ramakrishna -Calcuua
B. S. Chauhan -Narsinghpur
V. Ramakrishna Rao .. Regional Centre, Ministry of Environment &Forests Bhubaneswar.
Anthony Basil -Madurai
Abroad
M. Kawakatsu Biology Depu., Fuji Womens' College, Kiatku, Kita -16 Nishi Sapporo (Hokkaido) 00 1, Japan.
Jerzy Kolasa, Agrobiology Depu., Institue of Ecology, Suierczewskiego, 19-60, Poznan, Poland. Lcigth Winsor, Zoology Depu., James Cook University, Todnsville, Queensland 4811, Australia. T. K. Rucbush, Osborn Zoology Lab, New Haven, Connecticut, U.S.A.
Selected References
Hyman, L. 1951. The Invertebrates: Plalyheitninlhes and Rlzynchocoela Vol. II. McGraw Hill Book Co. Inc. New York. Whitehouse, R. H. 1918. Indian Land Planarians. Rec. Indian Mus.• 14 : 187, 16 : 29.
