108 



PLATYHELMINTHES 



[CH. 



contractile vesicle opening by a median posterior pore. In each pro- 

 glottis the trunks on each side are connected with their fellows on 

 the opposite side by transverse canals. The nervous system consists 

 of a ring in the head, whence two lateral trunks arise. There is no 

 brain the impressions the animal receives from the outside world 

 being few and simple. 



The reproductive organs have the same general structure as 

 those of the Trematoda. The vitellarium is however often unpaired, 

 whereas there are usually a pair of germaria. There is a large 

 uterus (8, Fig. 47) which appears to be a lateral outgrowth 

 from the conjoined oviducts. From the circumstance that in 

 primitive Cestoda this uterus opens to the exterior independently 



5 6 7 



Fig. 46. Transverse section through a mature proglottis of Taenla x about 12. 



1. Cuticle. 2. Long- necked cells of ectoderm. 3. Longitudinal muscle 

 fibres cut across. 4. Layer of circular muscles. 5. Split in the 



parenchyma which lodges a calcareous corpuscle. 6. Ovary. 7. Testis 

 with masses of male germ-cells forming spermatozoa. 8. Longitudinal 

 excretory canal. 9. Longitudinal nerve-cord. 10. Uterus. 11. Oviduct. 



of the genital atrium it is believed that it corresponds to the canal 

 of Laurer in the Digenetic Trematoda. If this be so the functions 

 of the genital atrium and Laurer's canal have been exchanged in 

 Cestoda, for in Trematoda the atrium permits eggs to escape 

 whilst Laurer's canal admits spermatozoa from another individual, 

 in Cestoda these spermatozoa enter through the atrium whilst the 

 canal is enlarged to form a uterus. The testes are in Cestoda 

 represented by a multitude of small rounded bodies (7, Fig. 46, 

 9, Fig. 47) from which excessively fine ducts are given off which 

 unite to form the single vas deferens ending in the penis. As the 

 proglottis gets older the eggs all pass into the uterus, which swells 

 enormously, displacing and destroying the other organs. In this 

 condition the proglottis drops off. This process is continually being 



