608 A. A. W. HUBEECHT. 



influence of natural selection acting upon the organisms that 

 have inherited this tendency in diff'erent degrees. Thus we 

 may understand the narrowing and lengthening of an animal 

 that moves in one direction in preference to any other ; and 

 similarly the development in the nervous system of a centrali- 

 sation not far away from the anterior extremity. 



All this has already been stated by Balfour in clearer terms 

 in his ' Comparative Embryology' (vol. ii, pp. 308, 311), where 

 he describes the gradual steps by which a radiate medusa-like 

 animal may have passed into a bilateral worm-like form, with 

 two longitudinal nerve-stems, which are regarded by Balfour 

 as the stretched nerve-ring of the Medusa. 



I fully endorse these views ; only, with respect to the nervous 

 system, I hold it to be safer to leave out of comparison the 

 already specialised nerve-ring of the Medusa, and rather to go 

 back to the Coelenterate nervous system as primitive as that of 

 the Actinia, where the plexus, both of the epiblast and the 

 hypoblast, with an increase in density in the region of the 

 mouth and the tentacles, may be said to be the fair representa- 

 tive of one of the lowest starting points. In this the plexiform 

 arrangement predominates. 



Now we find in all the lower invertebrates various though 

 distinct nerve tracts that are being specialised in this plexiform 

 nerve-tissue according to the modes of motion of the animal, 

 and according to the general shape of the body. 



Thus in the Medusae, which move about in the water by 

 annular contractions of the lower portion of the bell-shaped 

 body, one of the nerve-rings already alluded to was demon- 

 strated by the Hertwigs to innervate the musculature by which 

 this is brought about. 



In the Ctenophora the nerve system is less satisfactorily 

 known, but still Lang^ does not hesitate to bring them into 

 genetic relationship with the Polyclada. Among the latter. 

 Gun da, with its two longitudinal lateral stems, maybe looked 

 upon as an extreme term in this series. 



* A. Laug, 'Monographle der Polycladen,' Leipzig, 1884. 



