198 THE INVERTEBRATA 



The Cestoda as a group have feh the influence of the parasitic habit 

 more than the Trematoda. They have dispensed ahogether with a gut, 

 there is no mouth, and they absorb their food through the skin. As 

 they Hve always in the aUmentary canal of vertebrates they are con- 

 veniently situated for this purpose and the amount of food available 

 to them probably counterbalances the difficulties attendant on dis- 

 pensing with the usual method of digesting and assimilating food. 

 The ectoderm cells have sunk into the parenchyma after secreting a 

 cuticle as in the Trematoda, but this cuticle is thicker and divided into 

 layers. Immediately beneath the cuticle are the longitudinal muscles. 

 The circular muscles are incomplete at the edges. In transverse 

 sections the circular muscles appear to divide the parenchyma into 



ut. 



ect.c: cm. ;i„gc. 

 cut. I m.f{..; /^^_ ^_ 



In.excca- 



Fig. 156. Transverse section through a mature proglottis of Taenia, x about 

 12. From Shipley and MacBride. cut. cuticle; ect.c. ectoderm cells sunk into 

 the parenchyma; m.fi. longitudinal muscle fibres cut across; cm. layer of 

 circular muscles ; lime c. lime cell ; ov. ovary ; t. testis with masses of germ 

 cells forming spermatozoa; In.exc.ca. longitudinal excretory canal; In.n. 

 longitudinal nerve cord; ut. uterus; od. oviduct. 



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. 156). 



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 

 Elasmobranchs. 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. It is difficult from the structure to say 

 which end is the anterior and which the posterior, for the nervous 



