(PORK TAPE WORM)
PHYLUM : PLATYHELMINTHES
CLASS : CESTODA
ORDER : EUCESTODA
FAMILY : TAENIOIDEA
GENUS : TAENIA
SPECIES : SOLIUM.
Taenia solium is a common human cestode parasite . It lives in the intestine of people who eat pork. Hence it is called pork tapeworm. It completes its life cycle in two different vertebrate hosts , man and pig. Man is considered to be the definitive host and the pig is considered as the intermediate host.
Due to the infection of a parasite , host gets either complete or partial immunity. It may be obtained only while the parasite is present. This phenomenon is called premunition. Taenia solium induces premonition in the host.
EXTERNAL CHARACTERS :
The body of taenia solium is dorsoventrally compressed , ribbon like and elongated. Due to its tape like appearance , it is called tapeworm. The body is either grayish or yellowish or creamy white in colour. The length of the body varies between 2 and 4 meters. The body of the tapeworm is divided into three regions namely ,
- The pear shaped structure present at the anterior end of the body is known as the scolex.
- It is of the size of pin head.
- At the anterior end of the scolex , a conical , muscular structure called rostellum is present.
- Just below the rostellum , there are about 22to 32 hooks , arranged in two rows. The hooks are made up of chitin. The hooks in the upper row are larger , while the those in the lower row are smaller.
- Four round powerful suckers or acetabula are present , one on each side of the scolex, below the hooks.
- With the help of movable hooks , rostellum and suckers , the scolex of tapeworm is attached to the wall of the intestine in man. Hence the scolex is the holdfast organ.
The narrow short and unsegmented region behind the scolex is known as the neck. New segments are formed in this region and are pushed backwards . This process is known as strobilation. The growth of the parasite occurs in this region. Hence , the neck is called as the zone of segmentation or growth zone or zone of proliferation or budding zone.
- The entire , elongated and divided part behind the neck is called strobila.
- Each segment present in this region is called proglottid.
- The number of proglottids in tapeworm is about 800 – 900 .
- In Taenia solium , the proglottids are arranged serially one after the other , in the form of a chain , hence the species name solium ( solium; chain ) .
- In the strobila , old segments are present at the posterior end , while the new segments are added at the anterior end.
- The proglottids are attached with one another by the longitudinal excretory canals and nerve cords that pass through them.
- The division of body into segments in T. solium is not similar to the metameric segmentation seen in annelids and arthropods , which is a true segmentation. In animals that exhibit true segmentation , new segmentats are added from the anal segment .
- The segmentation in tapeworms is referred to as pseudometamerism.
- Based on the degree of development of the reproductive organs , the proglottides are categorized in to
- in the first 200 proglottides behind the neck , the reproductive organs are not developed . Such proglottides are called immature proglottides . They are wider the their length.
- The next 450 (201st to 650th ) proglottides possess the reproductive organs . Such proglottides are called mature proglottides . They are aquare in shape . Each mature proglottid has a genital papilla in the middle of the lateral margin . The common genital pore is situated at the tip of the genital papilla. These genital papillae are present irregularly alternate between right and left margins. This arrangement facilitates the exchange of gametes among different proglottides .
- About 250 ( 651st t0 900th 0 proglottides in the posterior end of the body are called gravid proglottides .They are longer than wide and are rectangular in shape. These are the oldest proglottides. Uterus is highly branched and filled with many eggs in these gravid proglottides.
- In Taenia solium , gravid proglottides are regularly detached from the posterior end of strobila and are sent out with the faeces of the host .The shedding of gravid proglottides by the tapeworm is called apolysis.
- Apolysis is useful to the parasite in regulating the length of the body and facilitating the transfer of gravid proglottides to the intermediate host.
BODY WALL :
The body wall of taenia solium shows
The tegument ,
The tegument :
- The outer most layer of the body wall is called the tegument.
- It is a syncytial cytoplasmic layer.
- It covers and protects the inner parts of the body.
- Several microvilli are present on its outer surface. They help in increasing the area of absorption of nutritive substances from the host.
- The tegument also shows several minute pores through which the predigested food is absorbed.
The musculature :
- Below the tegument , circular and longitudinal muscles are present . Tehse muscles are known as tegumentary musculature.
- In the mesnchyma of proglottid longitudinal , transverse or circular muscle layers and dorsoventral muscle fibres are present . These muscles are refered to as mesenchymal musculature.
- Alimentary canal is not present in the tapeworm. The space around internal organs in the proglottides are filled with mesenchyme. Hence , Taenia solium is referred to as acoelomate animal.
- Mesenchyme is differentiated into tegument forming cells and parenchyma.
- The tegument forming cells secrete tegument and maintain continuity with tegument through cytoplasmic processes.
- Parenchyma is a syncytial network formed by branched mesenchymal cells.the spaces in the network are filled with a fluid whose turgidity maintains the form of the body and acts as hydrostatic skeleton.
- Parenchyma also helps in the transport of substances to the tissues and packing the internal organs.