LONEAN SOCIETY OF LONDON. 1 9 



of motor nerve-cells. What intelligence Limulus has is confined 

 to its minute archicerebrum, and this is probably small in amount. 

 Amphioxus has no particular reason to fear Lhmdus on the ground 

 of brains. lu the Vertebrate the swellings of the nervous system 

 are associated with the development of large sense organs, but its 

 locomotor organs are the almost continuous bands of muscle 

 known as myotomes, and hence the motor nerve-cells form a practi- 

 cally continuous plate. Moreover, the whole study of the Animal 

 Kingdom is dead against the assumption that all else may change 

 but the nervous system must endure. If we start with the most 

 highly developed Arthropoda, or with the most highly developed 

 MoUusca, we find as we pass back to more primitive forms that the 

 nervous system evaporates into a mist of general ciliated nervous 

 ectoderm. Out of this, as required by the exigencies of motor 

 and sensory organs, accumulations of nerve-cells develop, and 

 disappear with the disappearance of these organs. Of course, 

 like every other organ, when they have persisted for a long time 

 in a phylum they become stable, but why we should trace the 

 highly developed brain of a Cuttlefish back to primitive ectoderm 

 and pass from the developed nervous system of a typical Arthropod 

 to the typical nervous system of a developed Vertebrate — ignoring 

 all the really primitive forms belonging to the Vertebrates, is 

 conceivable to no one who really knows zoology. 



The alternative theory to his, as Dr. Gaskell admits, is that 

 Vertebrates arose from some simple form with undiiferentiated 

 organs. Amphioxus gives us an idea of the Vertebrate structure 

 in its most undifferentiated form, but showing the characteristic 

 Vertebrate organs of notochord, gill-slits and tubular nerve -cord. 

 The worm-like Balanoglossus and its allies show the same 

 structures, but without the segmentation characteristic of the 

 muscles of Amphioxus and other Vertebrates. But in its develop- 

 ment, which shows far more primitive features than that of any 

 known Arthropod, Amphioxus resembles Balanoglossus. The larva 

 of Balanoglossus resembles that of Eehinoderms, and here we 

 have a hint of a wide ranging free-swimming group of pelagic 

 animals, the direct descendants of whom are Vertebrata, but the 

 degenerate off-shoof s of whicli at various levels are Echinodermata, 

 Enteropneusta, Amphioxus, and Ascidians. 



Dr. Gaskell heaps scorn on the idea that Vertebrates, the domi- 

 nant class, arose from a degenerate like Balanoglossus, and asks 

 how such worms could have competed with the big Arthropods. 

 No one supposes that Vertebrates are descended from Balano- 

 glossus, but at the immensely remote period of time when the 

 ancestors of Balanoglossus, leaving their closely allied compeers the 

 ancestors of Vertebrata, deserted the surface to seek the mud, 

 the ancestors of the Gaskellian Arthropods were probably in the 

 condition of the Trochophore larva. 



Dr. Gaskell alludes to Spengel's work on Balanoglossus a? 

 destroying the supposed Vertebrate character. Nothing coula 

 be more mistaken. Every argument o£ Spengel has been 



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