LIXNEAiSr SOCfETT OP LOXDOIf. 4 1 



In tlie Trematodes, inward migration baa taken place, and there 

 are six cords, two dorsal, two ventral, and two lateral, with a 

 network of connecting cords, some of which form a series of 

 hoop-like rings. 



In the Cestodes there is less inward migration, whilst there are 

 two lateral cords with occasional transverse connections. 



In the Nemertines, sometimes there is no inward migration, so 

 that the nerve-strands remain strictly snb-epidernial ; sometimes 

 the strands have completely separated. Tlie primitive continuons 

 sheath is frequently retained with two lateral and sometimes one 

 dorsal thickening. 



In the Nematodes also the extent to which inward migration 

 has taken place varies very much, in some cases the sub-epidermal 

 position being retained. Six strands occur in many forms, one 

 dorsal, one ventral, and two at each side ; these are connected by 

 traces of the primitive continuous sheath in tlie form of a very 

 broad anterior hoop, and narrow posterior strands. A different 

 arrangement of these antero-posterior strands occurs in front of 

 the nerve-collar. 



In Gordius, inward migration has occurred and there are three 

 ventral strands. 



In Arthropods, the inward migration and separation from the 

 epidermis are complete, and there are two ventral bands with an 

 anteriorly placed collar. 



In Balanoyloss^is, there is a continuous sub-epidermal sheath 

 which has not migrated inwards, and special dorsal and ventral 

 thickenings, and also in the collar region the very interesting 

 short neural tube with anterior and posi;erior neuropore formed 

 by invagination. 



In Chordates, there is a single dorsal band which migrates in- 

 wards, whilst the outgrowing segmental nerves may be taken as 

 specialized representatives of the continuous sheath. 



Erom the point of view of the general morphology of the 

 nervous system, therefore, the Chordate or Vertebrate group 

 exhibits simply one of a large series of different modes of spe- 

 cialization of the primitive diffuse, sub-epidermal sheath. 



In quite a number of these different experiments, the processes 

 of segmentation and of cephalization with the formation of a brain 

 have occurred independently, and have produced analogical or 

 homoplastic structures. The elaborate comparison of the results 

 of the processes of cephalization and segmentation in Ammocoetes 

 and higher Vertebrates with those of the Arthropods are meaning- 

 less unless we suppose that Ampliioxus has passed through such a 

 stage and has lost all traces of it ; it is a simpler supposition that 

 the higher Vertebrates have independently acquired the results of 

 cephalization after having passed through a stage of which Amphi- 

 oxus is the nearest living although specialized and degenerate 

 representative. 



As Prof. Gaskell has laid so much stress on comparison between 



