PRESIDENTIAL ADDEESS SECTION D. 191 



Science " present us with the most interesting examples that 

 have appeared of late of the class of biological theories con- 

 cerned with the relationships of organisms. Both deal with 

 the vexed question of the origin of vertebrates from inverte- 

 brates : one is entitled "On the Origin of Vertebrates from 

 Arachnids," and is by Professor W. Patten, known for his re- 

 searches on the eyes of Arthropods and Molluscs ; the other is 

 " On the Origin of Vertebrates from a Crustacean-like Ances- 

 tor," by W. H. Gaskell. 



Patten had been impressed by the necessity, in view of the 

 concentration observable in the vertebrate head, of looking in 

 the invertebrate ancestor of the Vertebi'ata for a similar kind 

 of concentration. This he finds in the cephalothorax of the 

 Arachnida. He then proceeds to point out a series of corre- 

 spondences between certain parts in living Arachnids, such as 

 the scorpions, and the vertebrates. These comparisons are 

 carefully and ingeniously marked out and illustrated. The 

 cerebral and infra-cesophageal ganglia of the scorpion are 

 shown to present parts corresponding to the fore-, mid-, and 

 hind-brains of vertebrates. The ventral chain, of course, cor- 

 responds to the spinal cord. The nerves given off from the 

 anterior ganglia are cranial nerves ; those given off from the 

 ventral chain are spinal nerves, and the latter are shown to 

 have dorsal and ventral roots. The sternal cartilage is the 

 primordial cartilaginous cranium of the vertebrate ; the 

 pectines are the pectoral limbs ; the pulmonary apertures the 

 gill-slits ; the sub-neural vessel corresponds to the notochord. 



To help out the resemblances between existing forms. Pro- 

 fessor Patten presses Ptericlithijs into thb service, as an 

 Arachnid which may have been on its way towards the verte- 

 brate type. But this hypothesis seems to have been suffi- 

 ciently disproved by Mr. Smith- Woodward in a note in the 

 " Annals and Magazine of Natural History " for October, in 

 which he points out that there are features in the dorsal shield 

 of Ptcricliihijs which indicate that it covered a vertebrate brain. 

 Thus, on the ander-surface of the plate between the eyes is a 

 depression identical in position with one which occurs in Coc- 

 costeus and other shield-beai'ing forms proved to have been 

 vertebrates by the discovery of the axial skeleton of the trunk. 

 And, again, as was originally shown by Dr. Traquair, the tail 

 in PtericJithijs is undoubtedly that of a fish. 



Gaskell had announced previously his view that Vertebrata 

 were derived from crustacean- like ancestors, and had sought, 

 and, as he thmks, found, confirmation of this view from the 

 results of a study of the brain of the lowest vertebrate that 

 possesses a brain — viz., the Avimoccetes larva of Petromyzon. 

 His conclusion is that the brain of the vertebrate is derived 

 from the cephalic stomach of the crustacean, with the wall of 



