V TREMATODA, CESTODA 171 



CESTODA 



The Cestoda or Tape-worms are like the Trematodes essentially a 

 group of parasites but they are marked off from the Trematodes by very 

 distinctive features, particularly by the elongated tape-like form of the 

 body with suckers or other organs of adhesion at the " head " end, by 

 the absence of an alimentary canal, and by the fact that the body 

 becomes subdivided into successive pieces known as proglottides which 

 recall the chain of sexual individuals of such a Polychaete as Myria- 

 nida, each proglottis possessing in itself a complete set of reproductive 

 organs. Further there is present a characteristic larval phase in the 

 life-history known as the cysticercus or Bladder-worm which inhabits a 

 different host from that inhabited by the adult. 



Taenia 



A typical Tape- worm of the genus Taenia (Fig. 78) possesses the fol- 

 lowing features. The body is greatly elongated and flattened except 

 towards the "head " end, and it is divisible into two regions — a short 

 scolex (Fig. 78 A, 5) with a rounded " head " by which the worm hangs 

 on to the lining of the intestine of the host animal, and a flattened main 

 portion formed of a chain of usually numerous — it may be hundreds — 

 of proglottides. The tip of the scolex is formed by a more or less pro- 

 nounced conical portion, the rostellum (Fig. 78, B and C, r), which at its 

 base widens out into the " head." At the periphery of the " head " 

 are four large rounded muscular suckers (Fig. 78, B and C, 5) while 

 arranged in a circle round the base of the rostellum are numerous 

 recurved hooks Qi). The shape and arrangement of these hooks show 

 characteristic differences in different species. 



The surface of the body is covered by a well-developed but permeable 

 cuticle through which the tape-worm absorbs nourishment from the 

 digested food-material of its host amongst which it lives. Remaining 

 as it does alive and healthy amongst the digesting food of its host the 

 tape-worm naturally remains quite unaffected by the digestive ferments 

 which bathe its surface. The internal organs are surrounded by a 

 packing of parenchyma, through which are scattered numerous small 

 rounded calcareous bodies. There is no trace of coelomic body-cavity, or 

 of alimentary canal, or of blood system. 



The nephridial organs are of the same type as those of the Trematoda, 

 a network of fine tubular channels traversing the parenchyma and 

 bearing scattered flame-cells. This excretory network drains into a 



