CESTODA 225 



sections the circular muscles appear to divide the parenchyma into 

 two regions, an outer cortical zone, where occur the cut ends of the 

 longitudinal muscle together with calcareous bodies, and an inner or 

 medullary zone, where the generative system lies (Fig. 162). 



The Cestoda may be divided into two orders : (i) Cestoda Monozoa, 

 (ii) Cestoda Merozoa. 



Order CESTODA MONOZOA 



These are small forms which live in the gut of fishes, usually Elasmo- 

 branchs. They resemble a trematode in shape and in the fact that 

 they do not form proglottides, but they have no gut. They have at 

 one end a "frilled" organ which serves for attachment, and a small 

 sucker at the other end. An example of this order is Amphilina. It is 

 difficult from the structure to say which end is the anterior and which 

 the posterior, for the nervous system consists of two cords running 

 down either side of the body with a single similar commissure at 

 either end. But when the animal moves it has the ''frilled" organ in 

 front so that is spoken of as the anterior end. 



Order CESTODA MEROZOA 



These are distinguished from the Cestoda Monozoa by the fact that 

 they all have the power of budding and so reproducing asexually, re- 

 sembling in this respect the turbellarian Microstoma lineare. The 

 adult worm has a scolex which is provided with organs of fixation 

 such as hooks, suckers or folds (Fig. 163). The scolex is usually 

 buried in the intestinal mucosa of the host. Behind the scolex 

 comes the neck, the most slender portion of the body, which may or 

 may not be sharply marked off^ from the scolex. It is in the neck that 

 asexual reproduction occurs, fresh segments being continually cut off 

 and, as they grow larger, pushed by the formation of new segments 

 away from the scolex. The segment so formed is called a proglottis. 

 The proglottis is not truly comparable with the new individuals pro- 

 duced in Microstoma lineare. Through each proglottis run the ex- 

 cretory canals and the nervous strands which are common to all 

 (Fig. 162). The proglottis when first cut ofiF from the neck region is 

 devoid of generative organs, but these develop as it becomes more 

 mature. When the generative organs are mature, fertilization of the 

 ova occurs, the ovaries and the testes disappear, and the uterus alone 

 remains to store the eggs. When the proglottis reaches this stage it is 

 "ripe " and breaks off to pass out with the faeces (Fig. 164). Despite 

 its connection with the scolex, each proglottis must be regarded as an 

 individual for it contains a full set of generative organs both male and 

 female. 



