102 



The embryos hatch out in water, and, having ciliated 

 coats, they swim about for a time. In the intermediate 

 host (a perch or some other freshwater fish) the embryo 

 becomes a plerocercoid larva (i.e., a solid elongated larva, 

 simply a scolex telescoped within its neck). If the fish, 

 raw or underdone, is eaten by dog, cat, or man, the larva 

 then develops (direct) into a tapeworm. 



Bothriocephalus lalus causes gastric trouble, sometimes 

 anaemia. 



PHYLUM NEMATODA (ROUND WORMS). 



Mention the Distinctive Characters of the Nematoda. 



The smooth white body is cylindrical in shape and usually 

 tapered more or less at both ends. The body- wall con- 

 sists of a tough outer cuticle, a syncytial ectoderm (i.e., 

 one in which the boundaries of the cells cannot be distin- 

 guished, a matrix of protoplasm with scattered nuclei), 

 and a layer of longitudinal muscles divided into quadrants 

 by four thickenings of the ectoderm which project in- 

 wards. The muscle cells are remarkable ; they consist 

 in greater part of unaltered protoplasm, only their outer 

 borders have contractile fibrils. 



The cavity of the body is not a true coelome. The ali- 

 mentary canal consists of a mouth, a short gullet or oeso- 

 phagus (stomodaeum), a long mid-intestine, and a short 

 rectum (proctodaeum) with a terminal anus. The main 

 nervous system is a nerve ring round the oesophagus, 

 with associated ganglion cells and a number of anterior 

 and posterior nerves. The excretory system is variable ; 

 .in many instances there are two lateral vessels which unite 

 anteriorly in a small bladder which opens to the outside 

 behind the mouth. 



The sexes are generally separate. The males, which 



are smaller than the females, often have the terminal 



portion of the body ventrally incurved. The reproductive 



organs are simple, tubular, and sinuous. The spermatozoa 



- are amoeboid. 



A noteworthy feature of the body tissues of Nematodes 



