i8o 



NEMATHELMINTHES 



becomes actively mobile. By means of a ring of hooks developed 

 round the anterior end it bores its way through the wall of the 

 alimentary canal, and after some time three weeks in E.proteus 

 comes to rest in the body-cavity of its host. By this time 

 most of the organs of the adult, with the exception of the repro- 

 ductive glands, are already well established ; the latter only attain 

 maturity when the first host is eaten by the second, and the 

 larvae find themselves in the intestine of a Vertebrate. 



FIG. 98. A, A larval 

 Echin&rhynchus proteus 

 Westrumb. further de- 

 veloped than ill Fig. 97. 

 Highly magnified. The 

 entoblast has developed 

 inside it the proboscis ; 

 b, b, the giant nuclei of 

 the ectoblast. B, The 

 entoblast at a more ad- 

 vanced stage, the ecto- 

 blast is not shown. The 

 outermost layer of cells 

 will form the muscles 

 of the body - wall ; the 

 body - cavity has ap- 

 peared ; a, proboscis ; 

 b, cerebral ganglion ; c, 

 body-cavity ; d, d, the 

 testes beginning to ap- 

 pear in the ligament ; 

 e, cells which will form 

 the generative ducts. 



Some of the details of the development are very remarkable, 

 and a short account of them may be given. The segmentation 

 of the egg is unequal ; it results in the formation of a central 

 biscuit-shaped mass of small cells and a peripheral mass of larger 

 cells ; the former is called by Hamann 1 the entoblast, the latter 

 the ectoblast. From the entoblast arise all the organs of the 

 body but the sub-cuticle and the associated lemnisci, which are 

 formed from the ectoblast. The latter has a remarkable history ; 

 the cells begin to break down and lose their outlines, whilst their 

 nuclei fuse together arid form a small number of giant nuclei, 

 which lie scattered throughout the syncytium thus formed. The 

 syncytium surrounds the entoblast on all sides ; by this time 

 the anteriorly-placed hooks have appeared ; in E. protein there 

 1 Jen. Zeitschr. Bd. xxv. 1891, p. 113. 



