CESTODA 



77 



that both processes have arisen from the 

 power of regenerating the necessary 

 organs in each of the new segments. The 

 result in both cases is the rapid forma- 

 tion of a number of joints, which gradu- 

 ally separate from the parent, to carry 

 the eggs and young to new stations. Just 

 as some Coelenterata (Lucernaria) may be 

 regarded as not having advanced much 

 l>eyond a scyphistoma stage, so there are 

 unisegmental Cestodes (e.g. Archigetes, 

 Fig. 37) which have remained as a 

 slightly altered but sexual scolex, directly 

 comparable with a Trematode, and, as all 

 authors are agreed, representing one gene- 

 ration only. Such monozootic forms are 

 now classed as a special family, the 

 Cestodaria or Monozoa, of which Caryo- 

 phyll&tt mutabilis, from the intestine of 

 various Cyprinoid fish, is the most abund- 

 ant representative, while Amphiptyches 

 (Gyrocotyle) urna, from Chimaera mon- 

 strosa of the northern hemisphere, is 

 paralleled by A. rugosa, found in Callo- 

 rhyiichus antarcticus of the southern 

 seas. 



Occurrence of Cestodes. The dis- 

 tribution of Cestodes and their larvae 

 is analogous to that of the digenetic 

 Trematodes, although the absence of an 

 alimentary canal limits the habitat of the 

 mature worms to certain sites, such as the 

 blood-vessels, the lymphatic and coelomic 

 spaces, and the digestive system, where 

 their body may be bathed by a nutritive 

 fluid. Almost all groups of Vertebrates 

 are attacked by Cestodes. Those of fishes, 

 and particularly of Elasmobranchs, are 

 distinguished by certain structural and 

 developmental features ; those of birds by 



FIG. 38. Scolex polymorphus 

 Rud. (larva of Calliobothrium 

 JUicolle Zschokke), from the 

 muscles of Apogon, a Medi- 

 terranean fish ; also found in 

 many Invertebrates (e.g. 

 Sepia). A, Inverted scolex, 

 with calcareous bodies ; B, 

 everted older larva. br, 

 Brain ; exo, terminal excre- 

 tory aperture ;/e.name-cells ; 

 for.sec, secondary excretory 

 pores ; hk, hooks of the adult 

 Cestode ; inrag, pit at the 

 bottom of which the head 

 is developed ; msc, auterior 

 sucker ; nl, lateral nerve ; sc. 

 suckers ; tl, tp, lateral and 

 main excretory vessels. (After 

 Mouticelli.) 



